Check for Commas

Could you check i have used commas correctly in following sentences?

1. I saw so many friends , that i knew from high school.

2. I often carry a cell phone with me , it helps if someone is trying to reach me.

3. If I had arrived earlier, I wouldn't have missed the opening act. 

4. Work was especially hard today , i had to train some one new to the job.

In the forth it seemed awkard could you also re-phrase the sentence also .

Waiting for your help !

thanks!

1. Before I could enter the building, I had to show my identification badges.Fine.

2. The car that was used as a demostration in the race, is owned by Toyota Motor Company.
Remove the comma if the subordinate clause is identifying information. If it is not, add a comma after 'car', and it will read better if you change 'that' to 'which'.'

It sounds a bit odd to 'use a car as a demonstration in a race'.

3. The tree, which has been part of the society for 75 years, was struck by lightning. Fine.

How can a tree 'be part of the society'? What does this mean?

Best wishes, Clive

Dear Mr X,

I am considering stepping down as a Parent School Governor.

Whilst the day-to-day running of the school and your future plans for Y enthral me, I feel I am not contributing in anyway to the meetings.

If you are happy for me to sit there listen and witness what goes on then I am content to remain on board. On the other hand if you prefer a more pro-active Parent Governor I will be willing to step aside to allow you to canvas for a more suitable candidate.