Learn Italian: Qui ci sono sette alberi grandi. - There are seven big trees here.

No, cardinal numbers like sette (seven) do not change their endings to agree with the noun. The only exception from 1 to 10 is uno (one), which changes depending on the gender of the noun. Numbers from 2 upwards stay exactly the same!

Why do we use ci sono instead of just sono?

By itself, sono means 'I am' or 'they are'. When you add ci in front to make ci sono, it changes the meaning to 'there are'. If you were talking about just one tree, you would use c'è, which means 'there is'.

Why is it alberi grandi instead of grandi alberi?

In Italian, adjectives that describe a physical characteristic like size or color usually come right after the noun they describe. Since grandi (big) describes the trees, it follows alberi.

Why does grandi end in -i?

In the singular, 'big' is grande. In Italian, adjectives that end in -e in the singular change their ending to -i in the plural, regardless of whether the noun is masculine or feminine. Since alberi is plural, we use grandi.