verbs and prepositions

[marker]

When we ask questions we have to use prepositions:

What are you interested in?

but what about the following phrases:

If you're interested, let me know.

If you're interested in, let me know.

?

Which one is correct?

[emsr2d2]

When we ask questions we have to use prepositions:

What are you interested in?

but what about the following phrases:

If you're interested, let me know.

If you're interested in, let me know. X

Which one is correct?

You need "in" if it's either followed by a noun or one is implied. With the question, "What are you interested in?", the logical answer will be "I am interested in + topic".

If someone says "If you're interested, let me know" what they are actually saying is "If you're interested in [topic], let me know". The omission of the topic leads to the omission of the preposition as well.

[marker]

Thank you very much :)

[Raymott]

When we ask questions we have to use prepositions:
What are you interested in?

...

That's a strange rule. Surely it only applies if there's supposed to be a preposition in the question sentence. And if there's supposed to be a preposition in the sentence, you put one in - in which case, what's the point of the rule?
There's no preposition in "What are your interests?" But it's a good question.
Do you routinely leave out prepositions from questions in Polish?