Why is depois pronounced with a regular d sound here? Before an e or an i, shouldn't it be a j sound?
The letter D sounds like jee when you have "di" or "de" at the end of a word
I have heard the jee sound at the beginning of a word sometimes like in the words "desculpa" and "demais". How do you know when you can say the jee sound at the beginning of a word?
It depends on the region you are. In the South/Southeast regions, you usually use "jee".
I wrote "I go for a walk after lunch." and it's wrong. Can any native english speaker tell me if i could say so?
You can say it. The English is not bad.
But in this case you are changing the verb while turning the original verb into a noun and adding 3 extra (and unnecessary) words to the DL translation from Portuguese.