Why is e4 so commonly played in the first move?

would help. Using an engine analysis would be better

If Black goes for the Old Sicilian, then it is Nezhmedinov-Rossolimo time.

yes i agree to all of you! but only when i play the alapin variation, usually i find myself with high chances of winning or sometimes i play 2.nf3. but if i play anything else, i usually lose. but an opponent knowing these both lines well will find it easy to beat me. so what to do?

What I usually do is study the openings that best suit you. And by that, i don't mean memorize long variations or turn the computer on to tell you what to do. But try to find books, pdfs, whatever that discuss the moves (as in, the purpose of the moves). And remember: if all else fails, just play moves based on the opening principles.

yes, good suggestion! thanks for it! i will do as you have suggested.

That's partially wrong. The books are written by people who are 2500-2600 players, and they also use the engine. You can use the Engine, but don't just memorize the opening, understand it. Understanding the Opening can tell you what to do if you forget a part of it. It also shows you the goal of that variation, and you can play the rest of the game accordingly and win.

Keep in mind, the Opening Principles can be ditched far enough into the game, and hyper aggressive players can ditch it if they see a promising attack

thanks

Personal preference.

But to answer directly:

Fights for the center.

Gains space.

Occupies a center square.

Controls a center square.

Activates the queen.

Activates the bishop.

I talk the talk, but I never walk the walk. That's why my ratings are so bad

Its like listening to my coach all over again...

Using computer without knowing basic stuff is bad in the same way as seeing the solution to a math problem is bad. It won't help you understand how to solve the problem (in our case, play chess), it will make you freeze when the opponent plays a different move than the one you are used to and it will leave you scratching your head planless.

In fact, spending time trying to learn a 20 move computer assisted variation in say, a Sicilian Najdorf will make you unable to beat an opponent that plays 1...a6*. If you know opening principles, that move should make you instantly relieved.

*Speaking about low rated players, so don't bring me the fact that f.e. Caruana does exactly that.

As a Sicilian player, I'm quite happy to see Nc3 and very pleased to see Bc4, I feel it gives me an easier game and my development is more natural. Bc4 in the open lines can cause problems to Sicilian players but in the Nc3 lines isn't very strong IMO. d3 is also bad since your basically increasing the power of g6 and Bg7 where the fianchettoed bishop becomes extremely strong.

Very famous game where tony Miles beat then world champion Karpov with 1...a6 in 1980 I think vs e4. 

Not 20 moves. The Sicilian is based to give Black a small advantage from the first 5 moves. Learning 10 moves of Closed or Anti - Sicilians is enough to equalize the game, or give White a minor advantage.

Yeah, I know that game, IIRC Karpov got cocky and paid for it in the end.

What advantage does black get in the first 5 moves?

The sicilian is blacks best chance to fight for an advantage against e4. However with accurate play this shouldn't happen. Certainly not after 5 moves! But 15 moves in with a couple of inaccuracies or mistakes thrown in and anything could happen.

Karpov didn't take Miles seriously, and got annoyed when he comfortably equalised and then made a serious mistake allowing Miles to win.

https://www.chessgames.com/perl/chessgame?gid=1068157

23.Qd3 is where Karpov went wrong. 

Did he lose because of 1...a6? No. 

Did it impact him in the game?  Yes.