- attendre: to wait
- atteindre: to attain
- étendre: to stretch
- éteindre: to turn off
Is there any trick to this pronunciation subtlety or is it just a matter of it becoming understandable over time?
You want to be thinking about the words in context rather than having perfectly distinguishable pronunciation between the words. The four words you have there are unlikely to be used in isolation, if you use them in a sentence people will understand what you are saying because they are surrounded by other words. The same goes with the duolingo activities where they say a sentence to you, many words sound similar (mais (followed by vowel) and maïs as one example) but you can work out what word is being used via the structure of the sentence as well as the other words around it.
It might also help to see how they are conjugated, you will find many more differences in pronunciation there. Hope this helps! Bonne chance!
If you want a series of specific words, you may use Google/Translate (their translations are not good enough, but their pronounciation is more reliable than on Duolingo). Just type those verbs on the French side, click on the small loudspeaker, listen then repeat with the voice, AGAIN AND AGAIN, until you feel that a) you are there; b) your mouth starts moving easily; c) you hear yourself enunciating all sounds differently; d) your brain has registered all these sounds and will recognize them next time you hear them.