Difference between イライラする and イライラしている

Question

I already know that the ている expresses a state after an action ended, rather than a continuous aspect in some verbs, but in some contexts I wonder why the natives would opt for する instead of している.

“イライラしている” means: “someone got angry and is still angry” “someone has gotten angry” “someone is irritated”

But if someone just says: “イライラする” what are all the meanings this sentence can convey depending on context? Here’s how I went about it:

• “Someone is getting angry” (hasn’t gotten angry yet, but might)
• “Someone will get angry” (when something happens)
• “Someone gets angry” (a habit)

Or for example in similar verbs:

へこんでしまっている
Someone has gotten depressed

へこんでしまう
Someone is getting depressed (has not gotten depressed yet, but fears might)
Someone will get depressed
Someone gets depressed

How can I express all these English tenses in natural Japanese?

Answer 1

Yes, all of those three are possible meanings of イライラする.

• イライラする!
  It's irritating! / It gets on my nerves!
• 彼の手紙を読んだら間違いなくイライラする。
  I will definitely be angry if I read his letter.
• 母は父の顔を見るといつもイライラする。
  My mother gets irritated whenever she sees the face of my father.
  _{(イライラしている is also fine; see: Habitual aspect)}

"イライラする!" is used typically when the speaker is already upset, but if I understand correctly, English speakers don't usually say "I'm being irritated!" when "It's irritating!" is enough, either.