[masaegu]

Originally Posted by Maxful

Hi masaegu san, could you elaborate a little bit more regarding why "は" cannot be used in those sentences?

I could but it might get you even more confused.

I don't know where and how you have been studying Japanese. Depending on that, what I'm going to say will probably throw you off.

If you learned は and が as some kind of a pair, please unlearn that. They function as completely different characters from each other in the language. When you replace one by the other, you will either have an utterly incorrect sentence for the context or change the meaning of the sentence.

「こちらが新入生の高田けんじ君です。」 = "This is our new student Kenji Takada"

The teacher is introducing a NEW student to the class. Takada's presence itself is "new information" for the class. The other kids never knew that they would have a new student until today, this moment. That is が.

When it's started raining suddenly, you say 「雨が降ってきた」. You NEVER say 「雨は降ってきた」. It's new information for the speaker.

You are walking home wanting to make ramen as soon as you get there. That's what you do almost everyday. You get home only to find out that there is no ramen in stock, which is very rare at your home. You say to yourself 「ショック！ラーメンがない！」　　We never, ever say 「ラーメンはない！」 in this situation.

Hungry and frustrated, you go turn on your PC to spam Japan Forum. However, you find the PC not working even though it was working fine in the morning. Now you know what you'd say here. 　「パソコン（　　）こわれてる！」
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In the sentence 「マックスフルさんが英語を教えてくれたので助かりま した。」, the reason to use が is quite simple. As a rule, the subject of a subordinate clause takes が.　 「マックスフルさんが英語を教えてくれたので」　is the subordinate clause of the sentence and the マックスフルさん is the subject of it.

スミスが行くならボクも行く。
お金があればニューヨークへ行きたい。
マックスフルが愛してくれないのなら、私はマサエグと結婚する！
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There are many other rules over the use of は　and が, but I'm not going to write a book here.