Hi,
Is it right when I write like this. If not, Please correct it. Thanks a lot.
1."They had a crop which is so good that they decided to have a party."

1."They had a crop which is so good that they decided to have a party."

Although the sequence of tense is a little odd (ie 'had[past] a crop' and 'is so good'), your sentence is correct. I suspect the best version for you would be:

"Their crop was so good, they decided to have a party."