Sure, you can say "He is nothing if not a good teacher." I found a few examples on the net: McCain is proving that he is nothing if not an independent-minded..... Obama is nothing if not glib. He is nothing if not a good performer. As Andrew stated, it means more than "very". If you say that the teacher is nothing if not a good teacher, what are you saying? The teacher obviously can not be a 'nothing', so he must be very good. It is an exagerration or hyperbole. The statement is not literally true; it is used to make a strong impression on the reader or listener. It stresses the fact that the teacher is "good". Other examples of hyperboles: My teacher is so old he personally knew shakespeare. I asked him a thousand times! I'm so hungry I could eat a horse. My teacher is so bad that I know less now than I did before I started studying English.

I dont think you would apply that expression to people, it may then imply the person is nothing (slightly insulting). It doesn't only mean very. It has implications of surprise that (for example) the holiday was more varied than expected.

yes, friend. you can say that!

This is a convoluted phrase. Be very careful in using it, because it has an implied meaning, not a 'face-value' meaning. For that reason, I would avoid using it for people unless I know them very well. And I strongly recommend hearing the sentence first before trying to say it. If you want help, post the request on rhinospike.com - I'll find it! Denis has given a good explanation of its meaning. :) Chopping up your sentence: "He is nothing / if not a good teacher" = "If he's not a good teacher / he is nothing" (which doesn't sound as pleasant, of course). So you see how the phrase has to be handled with care. Last piece of advice when using witty phrases: be prepared for a wittier answer! ;)

I agree with Denis. 'Be nothing if not...' is used to emphasise that 'someone' or 'something' has a lot of a particular quality (it could be used when talking about things or people). The expression means 'certainly', 'definitely' or 'very' and 'extremely': 'The holiday is nothing if not varied.' (= it is certainly varied) 'He is nothing if not a good teacher.' (= he's certainly a good teacher) 'It was the first time I had heard her sing, and I lay by the fire, listening and transported, for she was nothing if not an artist in everything she did, and her voice, though not strong, was wonderfully sweet and expressive.' The sea wolf by Jack London