Warning! I have deliberately chosen not to use the strict IUPAC names for the compounds in this section, because I see them as illogical and inconsistent with the older names for these compounds. The names for the compounds with one nitro group substituted into methylbenzene should be methyl-2-nitrobenzene and methyl-4-nitrobenzene. The reason for these names is that you should strictly name the attached groups in alphabetical order.
It seems to me to be illogical on three counts. Firstly, it breaks up the name of the hydrocarbon, so that it is no longer immediately obvious that you are talking about a derivative of methylbenzene.
Secondly, it doesn't relate to the older names for these compounds. Methyl-4-nitrobenzene, for example, used to be called para-nitrotoluene. The "para" refers to the 4 position. You could equally have called it 4-nitrotoluene. My name of 4-nitromethylbenzene is a direct translation of this. I think that's entirely logical!
Thirdly, you can end up with completely different names for compounds which are structurally similar. For example, if you think about substituting chlorines into these positions instead of nitro groups you will have to completely change the names for purely alphabetical reasons. That's silly!
I may well be in a minority of one on this, but I am sticking to my guns on it. In fact, IUPAC is much more flexible about these things than they are sometimes given credit for. I doubt if it would worry them, although it might upset your teachers or lecturers. Obviously, if you are working in a system which still calls methylbenzene "toluene", none of this bothers you!