"I will cut down the tree by a saw" is incorrect. You can say "with a saw" or "using a saw". The pattern "by + article + noun" is not generically used in the sense of "using" (possibly there may be certain idiomatic exceptions).
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kenny1999I will cut down the tree with a saw.Correct. Also,
kenny19991) I will do it myself 2) I will do it by myselfBoth 1) and 2) can mea
kenny1999DId you mean bothThe tree was cut down with a saw and The tree was cut down using a saw are right?Yes.
kenny1999It's also something I always want to know, what's the difference btw by using and usingYou will see "by using" and "using" used interchangeably. Personally I don't see any reason to say "by using",