AS

Like and as are often confused in English. They can both be used to talk about how things are similar.

Like is followed by a noun or pronoun. For example, "I'm like my sister", or "Like my sister, I have brown eyes."
As is followed by a subject and verb. For example, "She's a good student, as her brother was before her."
However, in spoken English, like is often used instead of as. "She's a good student, like her brother was before her."
As is used with a preposition, such as, "As in the 1960's, the population explosion will cause some problems."
We can use as in certain expressions, such as "as you know", "as you requested", "as we agreed".
We also use as…..as to give comparisons. For example, "He's as clever as his sister."

http://www.english-at-home.com/grammar/like-and-as/

used for comparing one person, thing, or situation with another

as...as:

Simon isn’t as tall as his brother.

I don’t play football as much as I used to.

I can’t run as fast as you.

just as:

We all need regular exercise, but a healthy diet is just as important.

half/twice/three times etc as much:

A house in London costs twice as much as a similar house in Liverpool.

the same as:

1 .Barbara’s suggestion was almost exactly the same as mine.

used in particular expressions for emphasizing a quality in someone or something

Her skin was white as snow. (Cildi kar gibi beyaz.)

as tough as old boots

2 spoken used for mentioning that something is already known or has already been talked about

As I was saying, we expect next year to be a good year.

As everyone knows, the farming industry is in serious difficulties. (herkesin bildiği gibi, ...)

3. used for saying that something happens or is done in the same way

I invested the money as you suggested.

Leave everything just as you found it. (herşeyi bulduğun gibi bırak.)

as usual:

Judith was late, as usual. (her zamanki gibi)

4. used for referring to what someone or something does or how they appear

As managing director, I am expected to provide effective leadership. (Yönetici olarak, ....)

5. used for saying that something happens at the same time as something else

As we were sitting down to dinner, the phone rang.

 (When  ya da while gibi kullanılır.)

6. used for giving the reason for something

As it was getting late, we decided to go home. (because gibi kullanılır.)

as for

used for introducing a subject that is related to what you have just been talking  (Yeni bir konuya geçerken. Mesela örnekte olduğu gibi önceden başkaları tasvir edilmiş. Ve sonra şöyle devam edilmiş. Çocuklara gelince, onlar...)

As for the children, they were happy enough to spend all day on the beach.

as if

1. in such a way that something seems to be true
   

   It looks as if it’s going to rain. (Yağmur yağacakmış gibi gözüküyor.)

http://www.macmillandictionary.com/dictionary/british/as