She used to let him ____ his money on whatever he liked.
a. spending
b. spend
c. to spend
d. spent

I think the correct answer is'' b. spend '' but my teacher told me that is '' d. spent ''
Well, I say b, same as you. "Spend" is an infinitive. "Spent" isn't.

But I'm always willing to learn new things. If your teacher can justify (d), I'd be really interested to hear how and why. Any chance you can ask?

I second that - Let him spendEmotion: smile
Students: We have free audio pronunciation exercises.
I agree. The construction is let + object + infinitive after the construction "used to". So actually there's no doubt.
teacher is definitely wrong on this occasion!
Must be the basketball coach teaching english again...
Teachers: We supply a list of EFL job vacancies
it is D because "used to" indicates she let him spent anything he wanted in the past.
another clue is the word "liked"....
hope this helps.
"Must be the basketball coach teaching english again..."
I must confess I laughed when I first read that. But it isn't funny at all. Actually, it is both sad and dangerous. What could you possibly teach anyone when you don't know the subject and you don't have the necessary tools for teaching either? In the end, the students lose, and that is really awful in the eyes or real teachers. Many of those who think of themselves as "language teachers" but who are not would probably do much better as basketball coaches.

The only right answer is b). "Let" (meaning "to allow or permit") is followed by a bare infinitive (the infinitive without "to"):
let somebody do something

The fact that "let" may be in the past, present or future doesn't make a difference:
- "She used to let him spend his money on sweets." (in the past)
- "She lets him spend his money on sweets." (habitually)
- "She will let him spend his money on sweets." (some time in the future)

Let me add a sentence

Here is a parallel of the structure for those who vote for 'd'

"...used to let him spent...." = ".....used to let him went..."

This explains that the second sentence is not correct.
Site Hint: Check out our list of pronunciation videos.