1
I need to focus on my homework.
Sign 'I', 'need', 'focus', 'my', 'homework'. When signing 'focus', direct the forward movement straight ahead, as if directing your attention to the imaginary homework on the desk in front of you.
2
Please focus on the teacher.
Sign 'please', 'focus', 'teacher'. You can angle the forward movement of the sign 'focus' slightly toward the actual location where the teacher is standing to show where the attention should go.
3
It is hard to focus here.
Sign 'here', 'hard', 'focus'. Pair the sign for 'focus' with a slightly frustrated facial expression or a slight head shake to emphasize the difficulty of paying attention.