1

× There was a traffic jam because the snow.
√ There was a traffic jam because of the snow.

We use because before a clause (subject + verb) and because of before a noun. Compare:

  • There was a traffic jam because it snowed heavily.
  • There was a traffic jam because of the heavy snow.

 

2

× I haven’t called because of I’ve been very busy lately. 
√ I haven’t called because I’ve been very busy lately. 

We use because before a clause (subject + verb) and because of before a noun. Compare:

  • I haven’t called because I’ve been very busy lately. 
  • I haven’t called because of my busy schedule.