which one is correct?
I am struggling with the superlative form.

You need to use the definite article when you express yourself in the superlative form, right?

This is the most interesting book in the library.

But how about the superlative of adverbs?

She swims fastest.
She swims the fastest

He speaks best.
He speaks the best.

The things I like most.
The things I like the most.

I had a look at my grammar book and found the following explanation:

"Highest degree: superlative usually with THE except when the comparison refers to one and the same person or thing."

So in the last example: "the things I like the most/the things I like most"

which one is correct?

Question

which one is correct?
I am struggling with the superlative form.

You need to use the definite article when you express yourself in the superlative form, right?

This is the most interesting book in the library.

But how about the superlative of adverbs?

She swims fastest.
She swims the fastest

He speaks best.
He speaks the best.

The things I like most.
The things I like the most.

I had a look at my grammar book and found the following explanation:

"Highest degree: superlative usually with THE except when the comparison refers to one and the same person or thing."

So in the last example: "the things I like the most/the things I like most"

which one is correct?

Answer

great question! the reason you need to use "the" in those sentences is because it is inferred that there is more than one person. She swims the fastest means that there is more than one person, infact it means that there are atleast three swimmers. If it was just her alone we would say "she swims fast" and if it was only her and another swimmer we would say "she swims faster" but since it is said "she swims THE FASTEST" we know it is more than 2 people. I know this can be confusing, hope I am not confusing you more! hahah 
As for the last part, I personally think that the correct one is "the things I like the most" if you are talking about more than two things, or for only two things (chocolate or vanilla) it would be "the thing I like better".

Answer

I swim the fastest

Answer

@Dark Panther it should be "I swim fast/faster" or you can write it like this " I am the fastest swimmer "
is that correct Mr. Adam81 ?

Answer

Pronoun + verb + THE best/fastest/most awesomely... I like it the most. But I do feel like sometimes when people speak fast, they'll leave out the "the".