Another green strategy is (to) use less ink, which is what many people already do.
In this sentence, do i have to use "to use" or is it okay to omit 'to' and just leave "use"?
Here you must include the “to” in order for the sentence to be grammatically correct. As a general rule, whenever a verb is directly following another verb, it will be in the infinitive form. The first verb in your sentence is “is.”
More examples: I want to go to the parade. She runs to stay in shape.
However, there are some situations where a “bare infinitive” is used, meaning that the “to” in front of the verb is omitted. Some common verbs that are followed by bare infinitives include can, could, may, might, must, should, will, and would.
Examples: He must receive a good grade. We will arrive at the beach before sunset.
I hope this helps you understand! Additionally, there are plenty of online resources that explain the other uses of the infinitive in-depth.
The "to" is always required to form a "to" infinitive, and so "to" cannot be omitted here. However, there are other structures that have the same form as the bare infinitive. One of these is the imperative.
It is possible that you could use a quoted instead of an infinitive phrase, and the quote could be an imperative sentence (as could be used to give instructions).
Another strategy is "use less ink", which is what many people already do.