Learn French: Ils ont un appartement près des monuments. - They m. have an apartment near the monuments.

The phrase for "near" is près de. When de is followed by the plural word for "the" (les), they must combine into the contraction des. You can never say "de les" in French!

Why is there a de (inside des) here at all? Doesn't près just mean "near"?

In French, the word for "near" is actually a two-word phrase: près de (literally "near of"). Because we want to say "near the monuments", we need près de + les monuments, which shrinks down to près des monuments.

Do you pronounce the 's' at the end of Ils in Ils ont?

Yes! Because Ils ends in an 's' and ont starts with a vowel, they link together. The 's' is pronounced like a 'z' sound. This is very important because it helps you hear the difference between Ils ont (they have) and Ils sont (they are, pronounced with a sharp 's' sound).

How can I remember if appartement is masculine or feminine?

The sentence gives you a clue: it uses the masculine word for "a" (un) instead of the feminine (une). As a handy rule of thumb, almost all French nouns that end in -ment are masculine.