What is future tense in english grammar?

English forms its "future tense" through the use of "will" (and in some cases "shall" - most commonly with questions in the first person)

For example:

Verb infinitive = "go"

I will go

You will go

He/She/It will go

We will go

You will go

They will go

Shall I go?

Shall we go?

Verb infinitive = "defenestrate"

I will defenestrate

You will defenstrate

He/She/It will defenestrate

We will defenestrate

You will defenestrate

They will defenestrate

Shall I defenestrate the cat?

Shall we defenestrate each other?

This works with every verb.