Prepositions before relative pronouns

Question

I've come across the following sentence:

think of how exciting it would be

Now i'm wondering if it's grammatically correct, because I've heard that a preposition can't be used before "that" and also I've seen similar sentences with "think how" without a preposition, like these which I took on the site with sentences examples.

So think how fast you can learn it.
Just think how Uther would react if he learnt that a serving boy had tried to poison his beloved ward.
Andrei, think how she must be suffering.

The question is if it's possib;e to put a preposition before a relative pronoun and if it change the meaning of a sentence. Thanks in advance.

BillJ · Accepted Answer

[1] Think (of) how exciting it would be.

[2] So think (of) how fast you can learn it.

[3] Just think (of) how Uther would react if he learnt that a serving boy had tried to poison his beloved ward.

[4] Andrei, think (of) how she must be suffering.

None of the above examples contains a relative clause (and none of the underlined elements is a declarative content clause) so "that" is not relevant here.

The underlined elements are all subordinate interrogative clauses (embedded questions). The preposition, here "of", is sometimes optional as it is in all your examples.

Sometimes the presence of a preposition can affect the meaning of the interrogative clause, but I don't think that's the case here.

Usually (but not always) the construction can be glossed with the formula "the answer to the question". For example [1] means:

"Think of the answer to the question 'How exciting would it be?'"

Prepositions before relative pronouns

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