Answers & Observations
I've noticed words like llover in Spanish that look similar in Italian (piovere) and French (pleuvoir). What's the relationship between these types of words, and what does this tell me about Spanish and Italian?
Let's keep the concepts of inheritance and sound change in mind. We're talking about correspondences identified through a process called, simply enough, "comparison" in linguistics. These are cognates in daughter languages derived from an ancestral parent language. As the Neogrammarians noticed over a century ago, sound change tends to happen regularly, operating on nearly every instance of a given sound in a given environment (so, speakers who "drop their aitches" often drop all initial aitches).
Specifically, these words share a similar shape because they are reflexes of a single late Latin word, and cognates of each other. That accounts for the similarities. I'll skip some of the complexities of the relationships between Vulgar Latin words and their cognates (although there's a writeup about Vulgar Latin sound changes on our old Romance Languages site here). The differences are due to regular sound change.
In the first set of words, you're seeing some of the outcomes of Latin initial pl-.
Spanish: llover (/j/)
Italian: piovere (/pj/)
Portuguese: chover (/S/)
The same rules also apply to initial Latin cl- (Vulgar Latin clamare > Spanish llamar (/j/), Italian chiamare (/kj/), Portuguese chamar (/S/)), and voiced counterparts bl- (*blondu > Italian biondo /'bjondo/), gl- (*glaciu > Italian ghiaccio /'gjattSo/), so we could say it's a "plosive + L" rule. In other words, it's the rule that determines the various outcomes (reflexes) of Latin words that start with a stop consonant followed by "l". In fact, if initial tl- and dl- existed in Latin, they would be subject to the same rule.
Consider another case: words that begin in f- in Latin. The outcome of Latin f- is a regular correspondence (that is, a regular sound change) that applies to non-learned words. Later medieval Spanish changed initial f to h (and eventually f > h > 0, or silence). The exceptions to this rule are learned Latin words,so we have Spanish hurto 'theft' with a corresponding learned word furtivo 'furtive'. We may choose to evaluate these as two separate cognates, one hurto deriving directly from old Vulgar Latin, the other furt- from the later acrolect. In fact, an Occitan dialect/language known as Gascon is undergoing the same change, and is in the intermediate stage in which initial h- is still aspirated (Vulgar Latin *fumu > Gascon hum 'smoke'). Other Romance languages mostly keep that initial f- (*fumu > Spanish humo, Gascon hum versus Portuguese fumo, Catalan fum, Sardinian fumu, Italian fumo, Romanian fum).
These sound correspondences are absolutely systemic in how they define the relations between individual Romance languages. For instance, how do the various languages above deal with final Vulgar Latin -u? What happens to intervocalic consonants (particularly voiceless plosives)? We can see the commonalities between words (especially when spelling obscures their differences) and also the way words changed within individual languages. Apply this at the level of grammar, or morphosyntax - what's similar about the verbs and nouns in the various Romance languages, for instance? Knowing that those, too, derived from Latin, what can we say about the common Vulgar Latin parent's noun and verb system? The same type of analysis applies there, too.
It's not a pointless linguistic exercise. Recognizing patterns in the Romance languages and the shape of Latin behind them makes languages that initially look so different as Italian, Spanish and French easier to learn in combination.
Romanian (Rumanian) has multiple reflexive pronouns ('se' & 'iși'). What's the difference between the two? Also, I understand that 'ei se spală' means 'they wash themselves', but why in the world does 'îmi citesc cartea' mean 'I read my book'?
Romanian has two sets of reflexive clitic pronouns and, so, two types of reflexive verbs: 1) the standard accusative type and 2) the secondary dative type. The dictionary form for type 1 is 'a se + infinitive' (a se spăla), while type 2 has 'a-și + infinitive' (a-și aminti). Type 1 comes from a Latin construction with a transitive verb plus the accusative (direct object) form of the reflexive pronoun 'sē'. Type 2 comes from a Latin construction with an intransitive verb plus the dative form of the same pronoun, 'sibi'.
First, let's avoid relying too much on the label "dative". Latin noun paradigms had distinct forms for the accusative, genitive (possessive) and dative cases. Through a process known as syncretism, Romanian speakers began to apply the original dative case to both uses:
grupul oamenilor 'the group of (the) people' (dative used for GENITIVE case).
a zis oamenilor '(s)he said to the people' (dative used for DATIVE case)
Speakers were so thorough in the way they syncretized Latin cases that we can simplify modern Romanian noun cases to 1) "unmarked" NOMINATIVE/ACCUSATIVE ('om' or 'om-ul') versus 2) "marked" GENITIVE/DATIVE ('omului').
Now, consider the same process applied to GENITIVE/DATIVE pronouns:
Îți citești cartea. 'You read your book.' (dative used for GEN)
Îți dai seamă... 'You realize...'; lit. 'you give to-yourself account' (dative used for DAT)
This contrasts with the straightforward use of the ACCUSATIVE reflexive (there is, of course, no NOMINATIVE):
Eu mă numesc Radu. 'My name is Radu'; lit. 'I call myself Radu' (accusative use for ACCUSATIVE)
The pronoun we designated "type 2" reflexive actually covers two underlying structures: type 2a, a dative reflexive pronoun and type 2b, a genitive reflexive pronoun. Same form, distinct usage.
So, what is the difference between the two sets of Romanian pronouns? The oblique form of Romanian nouns and pronouns is actually a "hybrid" GEN/DAT with two uses. Îmi, îți, își, etc can stand for 'of me/myself', 'of you/yourself', 'of oneself', etc. or 'to/for me', 'to/for you', 'to/for oneself', etc.
The "regular" reflexive, on the other hand, is accusative. Unlike nouns, pronouns maintain a three-way case distinction: 1 - NOM, 2 - ACC, 3 - GEN/DAT (vs. 1 - NOM/ACC and 2 - GEN/DAT for nouns). This doesn't count the separate VOC, or vocative in Romanian. Comparatively, this is unique to Eastern Romance languages; Western Romance (including French, Spanish and Italian) has done away with formal noun cases and further simplified pronoun cases.
How are book titles written in French? How about book titles in Spanish? Italian?
Here's a good rule of thumb for writing and recognizing French book and chapter titles:
1) the first letter of the first word of any title is capitalized
2) all subsequent letters of all subsequent words are uncapitalized (with the exception of proper nouns)
3) following punctuation rules, interrogative and exclamatory punctuation marks are removed one space from the final word of the corresponding utterance (C'est toi ? Mais oui !)
You will find some book titles capitalized differently. For instance, some conventions allow titles to capitalize key nouns, Guerre et Paix being the common example. Sometimes, publishers will capitalize every word in any left-branching nodes in the initial noun phrase along with the head noun (meaning article + any adjectives + noun, e.g. Les Trois Petits Cochons). Still, the rules of thumb above seem to be the most common.
Notice that on the spine of French books, the title and author are written from bottom to top (left to right if the book is face down), the exact opposite of English.
Spanish and Italian titles follow rules 1 and 2 above: Crónica de una muerte anunciada or Se una notte d'inverno un viaggiatore. Book spines in Spanish have the title and author facing the same way as French books, while Italian books have spine text facing the same direction as English language book spines.
How do you count ridiculously large Roman numerals (such as 525,600)?
This one's heading off topic, but it's relevant to Latin, so I'll give some quick feedback. I'll assume you know the basics, namely that you can count to 1,000 with Roman numerals.
There are extant accounts of people spending millions of sestertii ("sesterces" in English), and, by the late Empire, inflation had become a chokingly huge economic problem, so clearly the Romans had a reason to use large numbers. But, assuming they wanted to count such large numbers using their numeral system, and since they didn't have the Arabic system (borrowed from the Persians who borrowed from the Indians), how would they?
Really large numbers (4000+) may be written with a bar above, telling you to multiply the number by 1000. Given this principle, X stands for ten thousand, L for fifty thousand (L = 50), C for one hundred thousand (C = 100), D for five hundred thousand (D = 500) and M for one million (M = 1000).
DXXVDC = 525,600. (D = 500,000; XXV = 25,000; D = 500; C = 100).