I can't understand using of "ing" in this case
In my english practice I found this sentence: "I always enjoy chatting with you in the office"
It's correct, but I can't understand using of "ing" in this case. I would say "enjoy to chat". Could you please explain?

Question

Answer

Tom is correct that it's a gerund, but it has nothing to do with the action being completed. A gerund is a verb form that functions as a noun. For example you could replace the word "chatting" with "conversation" in that sentence: I enjoy conversation with you. The gerund names the action. The verb "enjoy" can't be used with the infinitive, so you have to use a gerund.

Answer

Anytime when the verb is the subject(noun) of the sentence or at the beginning of a sentence we add "ing"

Walking is good for you.

Overeating can be bad for you.

Swimming is fun.

Answer

This could very well be wrong, but this is what I think.

Short answer: enjoy takes -ing.

Long answer:

This is a place in English where there is no clear rule.  When to verbs are together in a sentence, some take the gerund, some take the infinitive, and some can take either.  For example, "want" takes the infinitive.

I want to buy
I want to go
I want to sleep
I want to run
I want to play

In contrast, "enjoy" takes -ing verbs.

I enjoy swimming
I enjoy biking
I enjoy chatting
I enjoy in-line skating
I enjoy sky-diving

So, this rule changes by word, and needs to be memorized.  Here's a link that might help.
http://www.englishclub.com/grammar/verbs-m_infinitive-ing.htm

Good luck!

Answer

It would be wrong to say "I always enjoy to chat with you". To chat is an infinitive form and chatting is a gerund. Gerunds are used when an action is already completed or done. When you say you enjoy chatting it is something you have already done, but to chat is something that you can do but haven't neccesarily done. For example: "I really enjoy swimming" and "I would like to learn to swim". I'm not sure exactly how to explain but here are two websites that will help you master the use of these forms: 
http://www.englishpage.com/gerunds/
http://www.ecenglish.com/learnenglish/lessons/gerund-vs-infinitive-practice
Good luck!

Answer

I like to chat with him. I like chatting with him. Both are fine. Watch out when you replace like with enjoy.  Some verb take gerunds, some take infinitive, and some take both. Do you want to chat with him?