Follow - up to my previous question

Bonjour Madame Cécile !

I thank you for reconsidering my query and helping me further.

Yes, Madame, the questions were answered perfectly but I require a few explanations on my questions-

1. “Il réalise que” as was mentioned in the right answer is “not followed” by Subjonctif Présent . I would like to ask “ How can one distinguish accurately whether an expression in the form ‘verb + que’ is followed by L’Indicatif /Le Subjonctif ?” . I wanted two-three examples which elucidate this subtle difference.

2. Monsieur Chris was right that the question was emphasised on the lesson - ‘être en train de’ and hence my answer was not accepted. 

I wanted to confirm whether the sentence - J’ai frappé la porte , mais tu dormais. was grammatically making sense or not.(which Monsieur didn’t highlight)

3. Ils doivent écouter — They must listen

Ils devraient écouter — They should listen.

Here, I wanted to know what is the characteristic of Le Conditionnel Présent which brings a distinction in the meaning of the two sentences.

Once again, thank you for your kind gesture.

Bonne journée !

Bonjour Madame Cécile !

I apologise for adding another follow-up but I just encountered a peculiar case with reference to Part 1-

J’estime que son témoignage est exact.
I consider that his testimony is accurate.

Now, here “ estimer + que” does not require the Subjonctif. But what is the deciding factor involved here ?

Will it also require Subjonctif in some cases as the verb to estimate sometimes depicts doubt ?