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Can I say,

(a) The owner of the car and the policemen thanked John for his help to arrest the thief.

(b) The owner of the car thanked two policemen and John for their help / for helping him to arrest the thief.
Comments  
... thanked John for his help in arresting the their. or ... thanked John for his help that led to the arrest of the thief.

(b) - The owner of the car didn't arrest the thief. This sentence is illogical. You can say: The owner of the car thanked the two policemen and John for their help.
for (b), I'm trying to say that, "the owner of the car thanked policemen and John for their help (because they help to arrest the thief), So, I continue to say, "....for their help to arrest the thief." Is that a wrong way to say?

IS there a incorrect to say, " thanked John for helping him to arrest the thief" ?
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The problem is not with the grammar, it's with the logic.

The POLICE (not the owner) arrest the thief. The OWNER can thank the police for arresting the thief, but not for their HELP in arresting the thief. If you say the owner thanked the police for helping him arrest the thief, it sounds like the owner arrested the theirf.
That means, if I change them to this way,

Aren't natural and logic to say,

(a) The owner of the car and the policemen thanked John FOR helping the police (them)to arrest the thieves.

(b) The owner of the car thanked John for helping the police (them) to arrest the thieves.

(c) The police praised John for his bravery to help them arrest the thieves.

(d) The police thanked John for helping them to arrest / thanked John for his help to arrest the thieves.
Dear grammar greek, or any other experts,can anyone giveme some comments?

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(a) The owner of the car and the policemen thanked John FOR helping the police (them)to arrest the thieves. Okay - I assume you don't mean to actually write the word "them." I usually do not use the "to" after help. Just "For helping the police arrest the thieves." This sounds like John was more active in the arrest than he was. He just called the police. He didn't chase the men or anything like that.

(b) The owner of the car thanked John for helping the police (them) to arrest the thieves.
Same comment about "helping them arrest the thieves" - He didn't play any real role in the arrest.

(c) The police praised John for his bravery,
which helped them arrest the thieves. Don't use "to" here.

(d) The police thanked John for helping them to arrest / thanked John for his help to arrest the thieves.
Same comments as above.

Try this:

The owner of the car and the police thanked John for his bravery, which led to the police arresting the thieves. (John didn't help with the arrest.)