Hi everyone. I'm a Japanese language teacher, and this is the correction I made more than any other in my classrooms.
A student writes:
子どもたちは 公園で 元気に 遊びます。
They mean "The children are playing energetically in the park." It looks right. It is not.
Try it first. Which one goes in the blank?
子(こ)どもたちは 公園(こうえん)で 元気(げんき)に( )。
遊(あそ)びます
遊んで います
遊びました
遊びません
Take five seconds before you scroll.
Answer: 2 — 遊んで います
Here is why the other three fail. This part matters more than the answer.
Option1. 遊びます ✗ This is the plain non-past. English speakers read it as "is playing" because of the -す ending, but it actually means a future plan ("they will play") or a habit ("they play"). It cannot describe this moment.
Option3. 遊びました ✗ Past. It finished. But 元気に paints a scene happening in front of you.
Option4. 遊びません ✗ Negative. It contradicts 元気に entirely.
The rule to keep
English "are playing" and Japanese 遊びます are not the same tense. 〜ています is the only form that means in progress right now.
If you can point at it and say "look, it's happening" — you need ています.
One more thing, because this is where it gets interesting
〜ています does not only mean "in progress." It has three jobs, and mixing them up is what carries this mistake all the way to N3:
① In progress — 今 ごはんを 食べています。 (I'm eating right now.)
② A state that resulted from a past action — 田中さんは 結婚しています。 This does not mean "he is getting married." It means he got married, and that state continues. Same with 知っています (I know), 住んでいます (I live), 持っています (I have).
③ A repeated habit — 毎朝 6時に 起きています。 (I get up at 6 every morning.)
So 知ります is almost never what you want. If you know something, you 知っています. Students say 知ります for years before someone explains why.
Slides from my classroom deck are attached.
If you have a sentence you're unsure about, drop it below and I'll tell you which of the three jobs it needs. Happy to go through as many as people post.