The phrase "it become difficult" is grammatically correct and can be used in written English. For example, "As I continued to make my way through the dense forest, it become difficult to see the path ahead.".
Also, if the problem scale increases, it become difficult to be dealt with even in the case of single objective because of the rapid expansion of the number of states to be considered.
It was only when a zoologist ranged over larger territories and found geographical variation that it become difficult to tell where, in terms of shape and color, one species ended and another began (p. 47).
The 'yes/no' motor-symptom questions addressed tremor-at-rest and balance: 1) 'During the last year, have you had tremors in one or both hands when at rest?' and 2) 'During the last year, has it become difficult for you to maintain your balance when walking?'.
At what point does it become difficult to hit the notes?
It becomes difficult.
It becomes difficult to maintain faith.
It became difficult to move.
Ludwig does not simply clarify my doubts with English writing, it enlightens my writing with new possibilities
Simone Ivan Conte
Software Engineer at Adobe, UK