I have been working through the exercises in Wenham's Elements of New Testament Greek and I'm not sure when to use δέ to translate "and" or "but", and when to use καί and ἀλλά. For example:
1. But God is rich in mercy.
I wrote Ὁ δὲ Θεός ἐστιν πλούσιος ἐν τῷ ἔλεϊ.
Wenham has put Ἀλλὰ (ὁ Θεός ἐστιν πλούσιος ἐν τῷ ἔλεϊ).
2. And he will have a voice like the voice of an angel.
I wrote Καὶ ἕξει φωνὴν ὅμοιαν τῇ φωνῇ ἀγγέλου.
Wenham has Ἕξει δὲ (φωνὴν ὅμοιαν τῇ φωνῇ ἀγγέλου).
3. And they will do wonders in his name.
I wrote Ποιήσουσιν δὲ τέρατα ἐν τῷ ὄνοματι αὐτοῦ.
Wenham has Καὶ (πράξουσιν τέρατα ἐν τῷ ὄνοματι αὐτοῦ).
I understand that δέ is a particle, lighter than καί and ἀλλά which are conjunctions. Could the words be interchangeable here, since these sentences stand alone? Does the distinction become clearer in the context of several sentences?