Learn Italian: I bicchieri sono puliti.

The singular is bicchiere (glass). In Italian, nouns that end in -e in the singular change to -i in the plural. This happens for all words in this group, regardless of whether they are masculine or feminine.

Because bicchiere ends in -e, how can I tell if it is masculine or feminine?

You generally have to memorize the gender of words ending in -e because they can be either masculine or feminine. However, in this sentence, the article I (the) gives the secret away: I is only used for masculine plural words, so we know bicchieri is masculine.

Why does the adjective puliti end in -i here?

Adjectives must match the gender and amount (number) of the noun they describe. The base dictionary form is pulito (clean). Since bicchieri is masculine and plural, the adjective changes its ending to -i to match, becoming puliti.

Why is the word for 'the' I and not Gli?

Both are used as the plural 'the' for masculine words, but the choice depends on the starting letter of the noun. We use I before most consonants, like the b in bicchieri. We only use Gli before vowels and a few specific consonant sounds (like z, or s plus another consonant).