Learn Spanish (Spain): El hombre lee muchos libros interesantes el fin de semana. - The man reads many interesting books on the weekend.

Muy means 'very' and is used before describing words. Muchos means 'many' or 'a lot of' and is used before nouns (like libros). Because we are talking about a quantity of books, we need a form of mucho.

Why is it muchos with an '-os' at the end instead of just mucho?

When mucho is used to mean 'many' or 'a lot of', it changes its ending to match the noun it goes with. Since libros is masculine and plural, mucho becomes muchos to match it.

Why is it libros interesantes instead of interesantes libros?

In Spanish, descriptive words usually come right after the thing they describe. Also, notice that interesante gets an -s added to the end so it perfectly matches the plural libros.

The English translation says 'on the weekend', but there is no Spanish word for 'on'. Why?

In Spanish, you don't use words like 'on' for days of the week or the weekend. You simply use the word 'the' (el). So, el fin de semana is the natural way to say 'on the weekend'.