Take a look at the sentence:
I has understood subject only after had watched this video. I had been thinking what it is something different.
So there is 3 successive time-points: think → watch → understand (all before "now")
What time I should use?
How does meaning changes for
The meaning doesn't change much with those verb tenses, but some sound more natural than others.
The first verb, "understand", only sounds right to me in the simple past ("I understood"). The present perfect "I have understood" would sound very formal or literary in this case ("Only after watching this video have I understood the subject." --Shakespeare 😄)
The second verb works best as the present participle "watching" (no pronoun) or the simple past "I watched". The past perfect "I had watched" isn't wrong, but it feels unnecessary.
The last verb I would conjugate as "I thought" or "I had thought". You could also say "I was thinking" or "I had been thinking", but again, it feels a little complicated.
Putting it all together:
I understood the subject only after watching this video. I thought it was something different.
