One of the challenges of learning a foreign language is that there are always a lot of verbs that have similar meanings. Even the briefest search of language forums reveals pages of people arguing over the minute differences between them. Of course, Japanese is no exception. Two of the most common verbs that learners find confusing are やる and する, the two words for ‘do’.
If you say both する and やる out loud to yourself, you should notice that やる has a slightly harsher sound than its cousin. Unsurprisingly, やる is usually considered a more informal version of する. やる is seldom used in any kind of formal situation or in written Japanese.
Grammatically, one of the major uses of する is to make nouns into verbs. So taking a noun like 勉強 (Study) as an example, it can be easily be converted into its verb form by adding する to make 勉強する (To study).
In a similar way, if I wanted to talk about tennis, I might say テニスをする (I play tennis). In this case, する is acting in a similar way to the English verb ‘to play’. While English often uses different verbs depending on what sport is being played such as ‘I practice Tae Kwon Do’ or ‘I go bowling’, Japanese simply uses する for all eventualities.
From here, things get a lot more complicated. One curious use of やる is that it is often used with things that are concrete, can be seen or has an effect in the real world. You would never hear a Japanese person connect words like 愛 (Love), 暗く (Dark) or 想像 (Imagine) with (を)やる, for example.
While やる can usually be considered as an approximate equivalent of the English word ‘do’, it also has another use as a way to put the speaker above the receiver of an act during an exchange. As a general rule this form is too harsh to be used with anyone that the speaker feels emotionally close to.
So while you’ll likely see 花に水をやる (Give the plants some water), you will rarely see a similar sentence about pets or children. One interesting exception is when someone is using a machine for giving food/ drink to their pet. These machines are still commonly called 餌/水やり用品.
Of course, while most owners view their pets as too much a part of the family to use a cold verb like やる, there are other animals that are not so beloved. やる is, of course, used when talking about ranch animals or animals breed for the diary industry.
After offering these general rules, it is worthwhile writing my usual acknowledge that, as with any vocabulary/ grammar point, it is worth going out and listening to how Japanese people naturally use both verbs in their day-to-day lives. Never has this been truer than with the verbs やる and する. Often one verb or the other is preferred simply because that is the way it is.
If you ask Japanese people why they say やった (I did it!) and never した; やれやれ (Go for it) instead of しろしろ; and why する is found in so many grammar forms such ～すればいい (Should~) or ～にする (Become~), most Japanese will simply shrug their shoulders and use another common する-form: しょうがない (It can’t be helped).