Quick question, important!
so I am trying to learn Chinese, and I do not understand why you would combine two of the same words to mean the same thing. Example:
现: now, at the moment 在: at the moment, now
现在: at the moment
打: to hit 开: to open
打开: to open
So why not use just one of these? Can someone explain me that? There are more examples, I will post if I find one.
This is not a complete answer, but better than nothing I hope. (and it is past midnight right now where I live, so not too long...)
First off, I'm going to draw a comparison with a couple of English words. Chosen off the top of my head, "cross-road" and "cupboard". If you look at these, you can see that both have individual words in there that relate to the meaning of the word, but that the actual word that you end up with has a different meaning. You can't say that instead of saying cupboard you should just say board, and still expect to end up with the same meaning. A comparable thing is going on with the Chinese (in your example of xianzai, for example).
Secondly, Mandarin has been historically going through a long shift from one syllable words towards two syllable words. Sometimes you would originally have had the same meaning with one syllable, but at a point in the past, another syllable has been added to make the word two syllables (the zi in zhuozi (desk, table is an example of this). Sometimes, as a result, a character has a meaning by itself, but realistically speaking you are only ever going to see it as part of a two syllable compound.
Thirdly, your example of dakai is a whole different issue. These are two verbs working together to get a resultative. In this case da alone means to hit or strike someone or something (or push in the case of switches or buttons). For example, for "I hit him (or her)" Wo da ta would be a perfectly acceptable translation.
However, adding another verb changes the meaning to the result of what happens. For example, if you beat someone to death it will be dasi (si=die); to turn something on ,i.e., dakai, you strike or push it until it is on or open (kai), and so on.