Questions & Answers about Můj syn dnes spí dobře.
Yes, Czech word order is fairly flexible, and all of these are grammatically correct:
- Můj syn dnes spí dobře. – Neutral: a simple statement of fact.
- Dnes můj syn spí dobře. – Emphasizes dnes (today), e.g. compared to other days.
- Můj syn spí dnes dobře. – Slight emphasis on dnes or on the combination spí dnes; still natural, though a bit less neutral than the first.
Czech tends to keep the subject–time–verb–manner ordering in neutral sentences, but shifting elements earlier in the sentence is a common way to emphasize them.
Because můj syn is the subject of the sentence, so it appears in the nominative case.
- Můj syn – nominative (who is sleeping?) → My son (subject)
- Mého syna – genitive/accusative (of my son / my son as an object)
- e.g. Vidím mého syna. – I see my son.
In this sentence, you are saying who is sleeping well, not referring to him as an object, so nominative (můj syn) is required.