請問for they are full of mischief and think it great fun to plague us中的it great fun是否要加上is?it is great fun.沒有be動詞這句怪怪的?謝謝?

"it"後面是不用加上"is".
這句的"think"解作"認為", 其用法和"consider"這字一樣, 在其受詞後一定要加補語(名詞或形容詞)以完成整個意思.
"think" + object + noun / adjective (called object complement)
1. think it great fun (noun) to plague us
2. think it enjoyable (adjective) to plague us

The  adj clause is followed by the no need to add "is". pattern.

The phrase "think" is described  as "imagine", which is used in the same way as the word "consider", and must be supplemented (noun or adjective) after its subject "they"to complete the whole meaning.  as a unit,  as object complement.
eg:-for they are full of mischief --------
and think it great fun/----obj complement as a unit
and consider it great fun----obj complement as a unit
to plague us.