So How Do I Say That in German?
Remember that, in German, the helping verb that means "will" is werden. If you want to say, "we will buy cheese tomorrow" in German, it looks like this:
Wir werden morgen Käse kaufen.
But let's say that you need that cheese before you do something else. Maybe you have some friends coming over for a cookout, and you need to go shopping first. Your friend calls to see if you have everything ready, and you say, "Don't worry, we'll have bought the cheese before you get here." The future perfect tense helps you write about two events in the future and tell the reader which one will happen first.
And so look at this English sentence:
We'll have bought the cheese before you get here.
In German, to write the future perfect, the helping verb werden works just like in the future tense. The helping verb haben (or sein, if it's an intransitive verb) goes at the end of the clause, and the participle goes right before it. Here's the sentence above, translated into German:
Wir werden den Käse gekauft haben, bevor du herkommst.
Note the difference between future (Futur) and future perfect (Futur II). In the future tense, the main verb appears at the end of the clause, in infinitive form; in the future perfect tense, the main verb appears in participle form, and is second-to-last in the clause, before either haben or sein, depending on the main verb.