What is "If you're tired" called in this sentence?

Question

"If you're tired, you should sleep."

What is the name for the phrase "If you're tired" in this sentence? Obviously "you should sleep" could stand alone as a sentence.

Answer 1

I prefer to call it an if-clause, or the conditional part.

I suppose you may call it the πρότασις if you’d really like to, but that’s a bit rich for my blood — even when transliterated into Latin letters as protasis.

Note that the ordering does not matter; the conditional part is set in bold in all these examples, no matter whether it comes first or second, and no matter its spelling:

• If you’re tired, sleep.
• If you’re tired, you should sleep.
• Sleep if you’re tired.
• I’d sleep if I were that tired.
• Unless I were incredibly tired, I wouldn’t sleep in my car.
• I wouldn’t sleep here unless I were dead-tired.
• Should you become too weary to drive safely, pull over immediately and take a nap!
• If Santa Clause was already here, then how come the chimney’s still plugged with that old crow’s nest?

For the protatically inclined, the other part of the conditional (that is, its consequent) is called the ἁπόδοσις, or if you prefer, the apodosis.

Answer 2

It's a subordinate clause. Refer to http://grammar.about.com/od/rs/g/subclterm.htm and http://oxforddictionaries.com/definition/english/subordinate%2Bclause .

Answer 3

I suggest calling the protasis the "condition," and calling the apodosis the "consequence."