Require is when you need something to do something. For example, a diesel engine requires diesel to run but it can also run on something similar for a limited amount of time OR Learning a language requires a lot of practice but it can also be learned through necessity. Demand is not correct to use instead of require. It is used when you are asking for something but not giving the recipient any choice. Demands contain ultimate authority while requirements can often be bent or modified somewhat. Demand can be used as a verb or a noun. As a verb: I demand that you stop right now. As a noun: The kidnappers have three demands.

Require is what you need in order to do something, or to fulfill a certain criterion whereas demand is when you ask someone to do something forcefully, not caring about whether they want to do it or not, and expecting them to do it whether they want to or not.

My friend!! I can’t send messages to you anymore! I don’t know why !! Please send me your WhatsApp number okay so I can text you in my WhatsApp okay!!

These words have many meanings and some of the meanings are very similar. This problem requires your immediate attention. This problem demands your immediate attention. The meaning of both is very similar. ‘Demands’ is a tad stronger.

Require means that something is needed. For example, you need fuel to make your car move, therefore, fuel is required for your car. On the other hand, demand means that it is not needed, but it is very much wanted. For example, gold is not needed to live as an individual, but it is pretty and shiny. Therefore, gold is in demand.