When I began my year of learning Italian, I was already a speaker of Spanish. And as both Spanish and Italian are descended from Vulgar Latin, they have a lot in common.
At times, having familiarity with Spanish was somewhat of an advantage, and at other times, it's actually a disadvantage. But a lot of the time, it's just a fact, neither a help nor a hindrance.
There are many ways in which Spanish and Italian are similar.
Both have a very similar Vulgar Latin grammar. Sentence structure is very similar. Both have two genders (masculine and feminine) and indicate them in similar ways.
In both languages, reflexive verbs have the reflexive pronoun on the end in the infinitive, and preceding in the conjugations. And except for a vowel change (si=se), the pronouns are basically the same.
Speaking of verbs, I find the conjugations remarkably similar between the two, throughout most of the verb tenses. Yes, they are different enough to necessitate study, but they are similar enough to make that study rather easy. Interestingly, I even found that irregular verbs are irregular in both languages, which must indicate a change that occurred long ago in Vulgar Latin.
Adverbs are formed in the same way between Spanish and Italian: simply add -mente to the end of the adjective. (eg: evidente becomes evidentemente).
With the exception of a minor spelling difference, the English -ity ending — which forms a noun from an adjective — sounds basically the same: in Spanish, -idad; in Italian, -ità.
In fact, there are a lot of spelling differences, and I actually count those among the similarities, because once you get used to those spelling differences — such as how Spanish makes the /kw/ sound with cu while Italian uses qu — you find that there is a lot of shared vocabulary.
There are also a few interesting differences.
Because most words end on a vowel in Italian, plurality is indicated with a vowel change, rather than by adding an s to the end, as in Spanish.
Spanish doesn't like words to start with an s, especially when the s is paired with another consonant (eg: español, estructura, escala). This problem doesn't occur in Italian, which happily begins words with consonant clusters starting with an s (eg: spagnolo, struttura, scala).
Because Italian has those consonant clusters, it also has a modification of the articles, to deal with them. Similar to a/an in English, the articles il and un become lo and uno in front of words beginning on such consonant clusters, to ease them out of the mouth.
And speaking of articles, Spanish counts possessives as articles, as in English, but Italian does not. Thus, in Italian you get some interesting constructs like il mio libro, or una mia amica.
Italian has the partitive pronoun ne, which I find both frustrating and amazing. Spanish has no such thing.
Both Spanish and Italian create contractions between prepositions and articles where the preposition ends on a vowels, but Italian also creates contractions (using the apostrophe) between articles and nouns when that noun begins on a vowel. And there is also freedom to create other, similar contractions occasionally. Spanish has no such contractions.
But perhaps the most significant difference — certainly the one that stands out to me when I hear the two languages — is the smooth, melodic, musical quality of Italian, with almost every word ending on a vowel. By comparison, a huge amount of Spanish words (half, if I guessed) end on consonants, which lends a slightly more harsh quality to the sound of language.
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I listed them here.