Question

I really couldn't meet up with u yesterday, I ____look after the neighbour's children. 1.had to 2.must to 3.have to 4.ought to

Answer 1

I really couldn't meet up with you yesterday, I had to look after the neighbour's children.

Among the choices we choose "had to" because the 1st clause in the given sentence is in the past tense.   "couldn't "  is the past tense of  "can't".   So the 2nd clause in the sentence also has to be in the past tense.

    "Must to"  is not a correct phrase.   "have to" & "ought to" are in present tense.  

Answer 2

"had to" as sentence is in past tense