Hi,
Here's your original question.
It makes my hair look presentable and neater.
Why is it hair look and not hair looks?
I said
Consider this example.
At 6pm, Mary decided to cook dinner.
My point is this. You don't add an 's' to 'look' for the same reason that you don't add an 'ed' to 'cook'.
In the above examples, both 'look' and 'cook' are infinitives.
Some verbs, like 'decide', require the infinitive with 'to'. Others, like 'make', require the infinitive without 'to'. But either way, it is an infinitive. Infinitives don't take inflected endings like 's' or 'ed'.
Does this answer your original question, or do you need more explanation?
Clive