WHEN IS TO A PREPOSITION AND WHEN IT'S PART OF THE INFINITIVE?

The -ing is used not only for continuous tenses, but also when you transform a VERB into a NOUN.

VERB ---> NOUN = VERB_ING

In the sentence:

"Studying is important."

Studying is a noun. 

Now it's easy to know when TO is a preposition or part of the infinitive.

Try substituting the verb for any noun.

>>If TO can be followed by a noun, it's a preposition, then use ING.

>>If TO cannot be followed by a noun, then TO is not a preposition, you need to use the infinitive.

TO + VERB in the infinitive

TO (preposition) + NOUN or VERB_ING

I am used to eat or eating the same foods? 

"I am used to the same foods" makes sense, it's correct English, then it means TO is a preposition, so you have to use the ING > I am used to eating the same foods. (present)

"I used to the same foods" makes no sense, it's incorrect, then it indicates TO is part of the infinitive, so you cannot use the ING > I used to eat the same foods. (past)

I am looking forward to seeing you. 

We use the verb in the –ing form (gerund) when it comes after a preposition.

 Use this formula:

PREPOSITION + VERB + ING

You will only be successful in the exam by studying hard.

I am interested in studying languages.

I use my computer for studying.

I use my computer to study. (Here, TO is not a preposition, but part of the infinitive).

She is used to going to the same restaurant.

(It means: she is accustomed, she has this habit. In "be used to", "TO" is a preposition, then use -ing)

She used to go to the same restaurant.

(It means: she had this habit, but not anymore. Here "TO" is part of the infinitive. Then, no gerund is used.)

(to be continued...)

I'm looking forward to seeing you?

seeing is a gerund preceded by a preposition...

These two articles may help you understanding the concept better.

https://www.italki.com/article/904/3-Wonders-Of-The-Gerund

https://www.italki.com/article/314/verb-patterns-in-english-gerunds-and-infinitives