Have you ever been to Japan? = (あなたは)にほんにいったことがありますか。(anata wa nihon ni itta koto ga arimasu ka?) I cannot speak Japanese very well. = わたしは日本語があまりじょうずにはなせません。 (watashi wa nihongo ga amari jouzu ni hanasemasen.) *You can also say: わたしの日本語はあまりじょうずではありません。(watashi no nihongo wa amari jouzu de wa arimasrn.) However, I would say: わたしはまだ日本語がじょうずにはなせません。(watashi wa mada nihongo ga jouzu ni hanasemasrn.) I hope this helps you.

Have you ever is "(phrase ending with past tense verb) koto ga arimasu ka?" Example: Amerika ni itta koto ga aru no? Have you ever been to America? "I cannot speak Japanese very well."- Nihongo dame desu or Nihongo jouzu jya arimasen yo. Your sentence is incorrect because you need to use "ni" after jouzu to use it as an adverb. The adverb making rule is: If verbs end in "i" remove "i" and replace with "ku". If verb is a na-adjective (such as jouzu), use the particle "ni" after it. 私は-remove this because the subject only included if it isn't obvious who you're talking about. 日本語を上手ni話すことができない。

まだ日本語はうまくないです = my japanese is not good yet this is better