adverbsgrammaticalityverbs
I'm not able to make it fly high.
If I change not able to unable, will it retain the same meaning? When should I use not able and when unable?
Is the phrase fly high correct? Suppose I'm trying to make a helicopter fly high, but it's not gaining altitude and I say "I'm not able to make it fly high." to mean that.
Both styles are used. In most genres, no-one will object to either. However, traditionally, first, secondly, thirdly etc. is used. Only pedants will insist on this usage, but it is something to be aware of, as there are many pedants. See Fowler's Modern English Usage (3rd edition).
The Oxford English Dictionary on firstly:
Used only in enumerating heads, topics, etc. in discourse; and many writers prefer first, even though closely followed by secondly, thirdly, etc.
Burchfield in Fowler's Modern English Usage on first:
"I don't understand" says that "I" can't figure it out - it puts the cause on you.
"That doesn't make any sense" says that your friend can't figure out how to explain it.
So the first wold be more polite.
There may be times when your friend actually doesn't make sense. Then you might say, "Could you please explain that again?".
Best Answer
As kiamlaluno says, using unable instead of not able doesn't change the meaning of the sentence.
But idiomatically, in virtually every context, it would be more natural to say "I can't [do something]".
There's nothing inherently wrong with "make it fly high", but I think native speakers would probably say...
The implication there being you've made it fly as high as you can. Very likely the reason you're saying anything at all is because it's currently (or was recently) that high, and whoever you're talking to knows how high that is/was. Otherwise, how does anyone know how high is "high"?