I want to know how to pronounce these sentences:

They develop the telephone.

They developed the telephone.

How can I differenciate the present form from the past one ?

I know that the context is important, but If I did not have context, how would I tell one from the other?

I appreciate your help.
They do sound quite different, but it might take a bit of familiarity to hear it.

developed sounds like developT

So the second sentence has a strong T sound followed by 'th' of the telephone.

The first sentence has no T sound between the p and the 'th' sound.
The first one "develop" is pronounced with an unreleased "p", the second is said with an unaspirated "p" followed by a glottal stop. The word "the" has nothing to do with the previous word and doesn't affect the pronuncation.