Learn Polish: Kupiłam gruszki i banany. - I bought pears and bananas.

Kupiłam is the past tense form used by a female speaker. If a man were saying this sentence, he would use kupiłem. In Polish, past tense verbs change their endings based on the gender of the person speaking.

Why is the word for "I" (ja) missing from this sentence?

In Polish, subject pronouns are usually dropped because the verb ending already shows who is doing the action. The -am ending on kupiłam tells us that "I" (a female) am the one who bought the fruit, so adding ja is unnecessary unless you want to emphasize it.

What grammatical case is used for gruszki and banany here?

They are in the accusative case because they are the direct objects of the verb (the things being bought). However, for plural non-human nouns like fruits, the accusative case is identical to the nominative plural (the basic dictionary plural form).

How do these words change from singular to plural?

A single pear is gruszka (feminine), and it changes the final -a to -i to become gruszki in the plural. A single banana is banan (masculine inanimate), and it takes a -y ending to become banany.