Um, well I suppose a lot are just two words that are commonly used together. Others are two words put together to get the meaning of both words are one thing.
*Japanese freak* Similiar is this concept.
? means mountain
? means sheep
?? means goat
So a "goat" would be a "mountain sheep". Japanese, as well as other languages, use compounds like this which I just thought might be useful. As you can see, the two together made a new word.
I think you just learn compounds. A lot of English words are compounds of words from other languages, often changed around a little.
There are three types of compounds. I think it might be confusing to use their names, so I'll just show you them.
#1 Compounds with two words together
#2 Compounds with two words separated (spaced)
- guinea pig
- water fountain
- air conditioner
#3 Compounds with dashes ( - )