In Italian, nouns are often used with a definite article more frequently than in English, especially when talking about something in a general or typical sense.
So Il kiwi è acidulo means something like Kiwi is tart or The kiwi is tart.
A few notes:
Acidulo means slightly acidic, tart, or somewhat sour. It often sounds a bit softer or more nuanced than acido.
Roughly:
So acidulo is a natural choice for describing the taste of fruit.
Also, acidulo agrees with il kiwi, which is masculine singular:
That a is there because of how piacere works.
Italian piacere does not work like English to like. A more literal way to think of it is:
So:
That is why the person who likes something is introduced by a:
Because piacere has a different structure from English to like.
Compare the two:
So in Italian:
And not something built like English mia sorella piace il kiwi, which would mean something more like my sister pleases the kiwi, and makes no sense.
Because piacere agrees with the thing that is liked.
Here, the thing being liked is il kiwi, which is singular, so you use piace.
Examples:
So:
With singular family members, Italian usually omits the article before the possessive adjective.
So you normally say:
Not usually:
A common exception is when the family term is modified or used in a special way, for example:
But in the basic form, mia sorella is the normal choice.
Italian word order is more flexible than English. The phrase a mia sorella is often placed before piace to make the structure clear and natural.
So this part:
is a very normal order.
You could also hear:
That also works. The difference is mostly one of emphasis and rhythm:
Yes, in a sense. Italian often leaves out subject pronouns or repeated subjects when they are understood from context.
In:
the subject of piace is still il kiwi, understood from the previous clause.
A more explicit version would be:
But that sounds unnatural. Italian normally leaves that pronoun out.
So English needs it, but Italian usually does not:
Normally, no.
You say a mia sorella, not alla mia sorella, because with singular family members Italian usually does not use the article before the possessive:
If you include the article, it is usually because the noun is modified or used differently:
So in this sentence, a mia sorella is the correct and natural form.