Learn Italian: Una mosca è entrata dalla finestra. - A fly came in through the window.

To describe a pesky, annoying fly, you can use the pejorative suffix -accio to make moscaccia. For a small fly, you can say moschina. However, watch out for the irregular moscerino: while it uses a diminutive suffix, it has become its own specific word meaning 'gnat' or 'fruit fly'.

If I add a diminutive suffix to finestra, does it just mean 'small window'?

It depends on the suffix and the gender! A finestrella or finestrina is indeed a small window in a house. But if you use the masculine diminutive finestrino, the meaning changes entirely: it specifically refers to the window of a vehicle, like a car, train, or airplane.

Why does the sentence use dalla (from the) instead of a word for 'through', like attraverso?

In Italian, verbs of motion like entrare (to enter), uscire (to go out), and passare (to pass) typically use the preposition da to indicate the point of passage or the opening someone goes through. While attraverso la finestra is also correct, dalla finestra is much more natural for everyday speech.

Just to be sure, entrata ends in -a because it agrees with mosca, right?