I don't understand the Alien he just seams to move at random just back and forth will not get out of my path

Throw a noisemaker or flare somewhere to grab its attention. Otherwise just stay crouched and do what you gotta do; long as you know to keep out of its sight--and make liberal use of the lean and motion tracker--you should be able to have a better understanding of your surroundings and know when it's best to move into the open.

@yummylee: ok cool thanks I was just not moving fast enough I find than when I gave a chance move as quick as I can

It also helps to get to know the sounds the alien makes when moving around, especially the sound of it entering and exiting a vent etc, if you wait around for it to enter a vent you can have a pretty easy job of moving around the levels, just keep an ear out for the sound of it coming back out.

Also a good tip is to always check vents when your moving around, if you see any vent with what looks like water dripping out its actually the alien drooling, if you wander under it your history, you can prevent afew annoying deaths just by keep an eye on that.

The alarm hacking to distract it is also really usefull in my experience.