Learn Italian: I treni e le borse sono qui. - The trains and the bags are here.

In Italian, the word for "the" has to match the gender and number of the noun. Treni (trains) is masculine plural, so it uses the masculine plural article i. Borse (bags) is feminine plural, so it uses the feminine plural article le.

How do the endings of treno and borsa change to become plural here?

Regular Italian nouns change their final vowel to become plural. The masculine singular word il treno (the train) changes its -o to an -i to make i treni. The feminine singular word la borsa (the bag) changes its -a to an -e to make le borse.

Why do we use sono instead of è in this sentence?

Sono is the "they" form of the verb essere (to be). Since we are talking about multiple things together (the trains and the bags), the subject is plural and takes the "they" form. If we were only talking about one single train, we would use è (is).

Could I leave out i and le and just say "Treni e borse sono qui"?

No, you usually cannot drop the articles in Italian. While in English we often skip "the" and just say "Trains and bags are here," Italian almost always requires definite articles for the subjects of a sentence.