No. He understood why I was crying is grammatical. How do you translate it into Czech?
grammatical? This sentence is translated in the title of the discussion. I am not sure what you are asking about.
And why not "He understood why I am crying"? Shouldn't it be jsem plakal for was crying?
Chápat impf -> pochopit perf
The question i have is when do you use present, imperfect or perfect in reported speech. In this example you appear to be using the present tense placu and translating it into english imperfect.
Also it seems in the first phrase you are using the perfect tense together with the present tense in the second phrase. Is this always the case?