so わかる generally means "to understand" or "to know"
わかっている is the progressive form of わかる
わかった is the past tense of わかる
For example, you're watching tv and your mom tells you that you need to do homework.
So you want to answer to her that you "understand."
You tell her: わかっている！
This means that you know this already. (you already understood that you needed to do your homework but you were not doing it) This response would roughly translate to " I know that already." You already knew or you had currently been in the state of knowing this. (either way, in this particular moment of time, you are in the state of understanding/knowing)
You tell her: わかった！
This means that you understand, now that your mom has told you. This response would roughly translate to "I got it." It's in that particular moment that you understand it.
To use the former person's example: about the apple
So, I'm explaining to you that I'm eating a fruit. I don't know the French word for "apple," so I'm saying "it's red, crunchy and sweet."
So you understand that it's an apple.
りんごだとわかっている。You already know that it is an apple (maybe because I love apples and am always eating an apple when I'm talking to you) OR You're listening to my explanation and you are in the state of knowing that it is an apple.
りんごだとわかった。Through my explanation, you now understand that it is an apple. This implies that there is a moment in which you say "ah! it's an apple!"
Sorry for the clumsy explanation. Hope it helps in some way!
Ex: I understood about apples in your explanation.
Ex:I know about apple.
From recently to present→わかった
I am sorry that my English is not good.
I’m not an expert but from my limited understanding.
分かる(わかる) – dictionary form of “to understand”
分かった(わかった)= “did understand” - past tense
分かっている(わかっている)= “currently understanding” - present tense
Hope that helps
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