need 後面幾時不加 to

I don't think we need (to) ask him.
The word (to) is definitely necessary here.

簡單0的講
當 need 係指需要做一件事時, 要 to
當 need 係指個人需要一件物品時, 唔駛to

grammatically,
當 need 後面係 verb, 要 to
example: I need to go to school. I need to call him back.
當 need 後面係 noun, 唔駛 to
example: I need money! I need you. I need water.

need - (1) 助動詞; (2) 一般動詞
(1) 助動詞 (modal verb) need + verb
表示加重語氣, 是指在那 "情況" 下, 需要去做,
(2) 動詞 need + infinitive verb
表示 "行為" 上須要以下行動

一般來說
need後面如跟動詞需要加上to
後面是名詞就不需要
如
I need to go to the supermarket to buy some groceries.
I need a screwdriver to fix this chair.