Problem with article "the"

I don't know when to use article "the", because sometimes it sounds like "da" and sometimes sounds like "di". I'll give you an example:

The man
The apple

The article "the" sounds different. Please, help me to understand.

Regards,

Naif Salomón

The pronunciation, "tha" is the general sound.  When you use the 2nd one, it means to give greater emphasis to the object you are talking about.  In writing, they mean the same thing.  In another words, no difference at all.  This difference is only used in speech.