Learn Russian: Это мой любимый фильм.

In Russian, the present tense of the verb 'to be' is almost never used. When saying 'This is [something]', you simply put Это right next to what it is, with no connecting verb.

We are learning about нравится (to like). How is любимый different?

нравится is a verb used to say that something pleases you, as in Мне нравится этот фильм (I like this film). любимый, on the other hand, is an adjective meaning 'favorite'. You use it to directly describe a noun just like any other adjective.

Why does the sentence use Это instead of the masculine form Этот to match the masculine word фильм?

When you use 'this' to introduce a completely new thing (meaning 'This is a...'), you always use the fixed word Это, regardless of the noun's gender. You only use the masculine Этот if 'this' points directly to the noun, like in the phrase Этот фильм мой (This film is mine).

Why do мой and любимый have different endings if they both describe the same masculine word?

They belong to different grammatical groups, so they follow different spelling patterns. мой is a possessive word (my), while любимый is a standard adjective (favorite). However, both are currently in their masculine singular forms to perfectly match the masculine noun фильм.