Using は and が properly is not a true problem.

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Oct 5, 2011 11:03 Japanese_language 「が」 「は」 「も」 「ない」 助詞
Many people are interested in how to use は and が properly, but they, as well as native speakers, do not use は before ない properly. Actually, は and ない have a link with each other. Without は, ない often becomes vague and your sentences depend on the context, which often makes misleading.

彼は慎重にそれをしなかった。
This sentence is grammatically correct but vague.

彼は慎重にはそれをしなかった。 (は added)
He didn’t do it deliberately.

彼は慎重にもそれをしなかった。 (も added)
Deliberately he didn’t do it./Since he was deliberate, he didn’t do it.

The one with は is the same as:
彼は慎重にそれをしたわけじゃない。
それをする際,彼は慎重さを欠いた。

は/も and ない have a link with each other, so the postpositions は and も are called linking particles, not case ones. To the contrary, が is a case particle. To tell the truth, talking about how to use は and が properly is odd because they are quite different parts.
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