It just be two because it would just look weird as one(headstart) but you could make it one with a hyphen(head-start) that would also look good. It depends on how you are using it. If you use it in a sentence, such as, "I got a head start in the race today," it is written as two words. If you are using it as a company or program name it depends on how they choose to write it.
La cabeza
Off the top of my head, I believe that I can.
It should start with (mis) :)
The word "hairbrush" is typically written as one word, not two.
No, you should not. You should only capitalize a word when it is at the start of a sentence or when it is a proper noun.
no
Written as one word (hogshead) it is a very large cask (wooden barrel)
Headset, one word.
The word "meerkat" should not start with a capital letter unless it is at the beginning of a sentence or part of a title. It is a common noun referring to a species of small mongoose found in southern Africa. Therefore, in most contexts, it should be written in lowercase.
Generally, it's one word.
However. On the other hand.
Underdeveloped should be written as one word, not two words or hyphenated. I hope this answered your question. :)