1. He held a party for his kid's birthday.
2. He held a party which was for his kid's birthday.
3. He held a party to celebrate his kid's birthday.
4. He held a party for his kid.
5. He held a party which was for his kid.
6. He held the party which was for his kid.
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In #2, if we delete 'which was', we can get #1. Right? So can we say that #1 is from #2? Can #5 and #6 be #4?
Which one is correct, #5 or #6?
These are fine:
1, 3, and 4
The others aren't incorrect, but they are wordy and probably would not be used.