Re: Tsuyoku naritai
I think that tsuyoiku naritai fits when talking about wanting to become better at aikido.
I remember talking to sensei about this a couple of years ago.
If you think that someone has a strong aikido in a positive sense, you use the word tsuyoi. But if you think that someone has a strong (hard) aikido in a negative sense you can use the word katai.
If you want to complement on someone's aikido as being soft in a good way yawarakai is a good word. If you on the other hand want to describe it as soft (powerless) in a negative way the word yowai is the word to use.