Waist

Our waist help our legs and our upper body to be connected.

And our weast help us to twist.

Hair

Our hair help our head skin to be warm.

Elbow 

Our elbows help us to grab something easier. Like if our arms are just straight, then it will be hard to grab stuff.

Knuckle

Our knuckles help us to reach to the sky.

Our knuckles help our hand and arm to be connected.

Correction:  Student's sentence   Corrected sentence        Grammar explanation       Better 

Waist

Our waist help our legs and our upper body to be connected.

Our waist helps connect our legs and upper body.

The word 'help' is singular here because it refers to the collective action of the waist, which is a singular entity. To be connected has been simplified to "connect" for clarity and conciseness.

And our weast help us to twist.

And our waist helps us to twist.

We change the word ‘weast’ to ‘waist’, which refers to the parts of the body. The singular form of "help" is used here because it denotes the action carried out by the singular entity, the waist.

Hair

Our hair help our head skin to be warm.

Our hair helps to keep our scalp warm.

The word ‘helps’ is used because it agrees with the singular subject ‘hair’. The phrase ‘to be warm’ is simplified to ‘keep....warm’ for clarity and conciseness. And lastly, the noun phrase ‘head skin’ is replaced with the word ‘scalp’ for clarity and accuracy.

Elbow 

Our elbows help us to grab something easier. 

Our elbows help us to grasp things more easily.