Learn Italian: I bambini odiano il pesce. Non lo mangiano. - The children hate the fish. They do not eat it.

In Italian, direct object pronouns like lo (meaning 'it' or 'him') usually go immediately before the conjugated verb, rather than after it like in English. So instead of saying 'They do not eat it', Italian phrases it as 'Not it they eat' (Non lo mangiano).

How do I know to use lo here instead of la?

The pronoun lo replaces il pesce (the fish). Because il pesce is a masculine singular noun, you must use the masculine singular pronoun lo to say 'it'. If they were avoiding a feminine noun like la verdura (the vegetable), you would use la instead.

The word pesce ends in -e, so how can I tell it is masculine?

Nouns ending in -e in Italian can be either masculine or feminine, so you eventually have to learn the gender with the word. However, looking at the article before the noun is a great trick! The article il is strictly masculine singular, which gives away that pesce is masculine.

Why doesn't the second sentence start with loro (they)?