The phrase "I must help" is correct and usable in written English.
You can use it to express a sense of obligation, for example: I feel a strong sense of responsibility to those in need, so I must help.
For my father and my brother it's too late; but I must help my mother stand up straight".
The young man added, "He told me there was a way of helping them and that I must help them".
If I am to grow good corn, I must help my neighbors do the same".
"I thought I must use it in a way which suits her - I must help other people".
"When I started going to the schools and I started this programme, I said I must help them as much as I can.
What do I know -- this was my very first marathon, but I think it must help when you have a real connection to the city you're running in.
"I suppose I just must help Navalny and his team to win," said Ivanov, who spent his first day carrying boxes.
"I thought, 'We must help them.'".
It must help, I suggested, to feel the audience on your side, as it surely was.
"Tota, come and help me, I am afraid, you must help me.'" What none of us can say, or even begin to understand, is how Messi is coping behind that inscrutable face.
