No, you should try to master the correct sound of the hard and soft "th" consonant. If you just can't get the "th" sound, then of course do the best you can. But "d" is not a good substitute and it is considered to be a mispronunciation.

Here is a podcast you might be interested in: http://acupofenglish.com/pronouncing_the_th_ As far as I am concerned, it helps me a lot when I was struggling with this “th” sounds.

No; you cannot substitute a "d" or "t" for "the". You cannot pronounce "that" like "dat" or "this" like "dis" if that is what you mean by your question.t

No. Don't substitute sounds. You'll only sound like a poor speaker (or worse, you'll sound comical and childish). This applies to any language. The "th" sound is really not that difficult. Stick your tongue between your teeth and go! Look in the mirror: if you can't see your tongue, then you're pronouncing "th" wrong. You can always ask a native speaker to show you how it's done.

It's a "d" sound. I pronounce it slightly softer than a regular d in other words though because of what I've heard from other people but it's basically a "d" sound.