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The teacher helped/ had helped him, otherwise they would never have been able to solve the problem.
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wangqh2696122 The teacher helped/ had helped him, otherwise they would never have been able to solve the problem.
This is really two sentences, and it's fine with or without 'had'. I think I would use 'had'.

The teacher (had) helped them. Otherwise they would never have ... problem.

CJ
Comments  
wangqh2696122 The teacher helped/ had helped him, otherwise they would never have been able to solve the problem.

wangqh2696122,
You should pay attention to the use of pronouns. You have him / they mix up.

Your sentence has issues. If you want to use the conditional structure, your need to say:
3rd Cond: If the teacher had not helped him, he would have never been able to solve the problem.
2nd Cond: If the teacher did not help him, he would never be able to solve the problem.

Common English: Both versions are fine.

He would never be able to solve the problems if not for the teacher's help.
Or, If not for the teacher's help, he would never be able to solve the problems .
Students: We have free audio pronunciation exercises.
By the way, should we say "He would never have been able to solve the problems were it not for the teacher's help." or ""He would never have been able to solve the problems had it not been for the teacher's help."?
wangqh2696122""He would never have been able to solve the problems had it not been for the teacher's help."?
This is ok.
 CalifJim's reply was promoted to an answer.
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