Yes, you're right: "p" and "t" have "explosive" sounds, but "b" and "d" have softer sounds. "t" and "d" are pronounced by touching your upper palate with your tongue, but the contact is stronger with the "t" and there's a short instant in which the air is stopped behind the tongue. With the "d" sound, the air doesn't have to be necessarily stopped (Spanish) To pronounce "p" you have to put your lips together in order to stop the air and then release it. To pronounce "b" you don't have to stop the air the same way, and your lips' contact doesn't have to be so tight. Please tell me if I made any mistakes writing in English. I hope the explanation is not confusing and it helps.

This is a very common problem for native Arabic speakers. The main difference is that /p/ is aspirated (= you quickly push air out of your mouth when you say it) and /b/ is not. Put your hand in front of your mouth and say these two words: pear bear If you say pear correctly, you will feel air hitting your hand. You will not feel this if you say bear correctly.