δέ vs. καί and ἀλλά

[LeoC]

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?

[modus.irrealis]

I wrote Ποιήσουσιν δὲ τέρατα ἐν τῷ ὄνοματι αὐτοῦ.
Wenham has Καὶ (πράξουσιν τέρατα ἐν τῷ ὄνοματι αὐτοῦ).

Just in case it's not a typo, ὀνόματι.

I understand that δέ is a particle, lighter than καί and ἀλλά which are conjunctions.

I know some sources call it a particle, but in this use, it's no less a conjunction than καί and ἀλλά. It's just postpositive.

Could the words be interchangeable here, since these sentences stand alone? Does the distinction become clearer in the context of several sentences?

Yes, exactly. Without a larger context it's impossible to say that there is only one right answer. The distinction is something you'll pick up as you encounter it more often. I wouldn't say, though, that δέ is lighter. Basically Greek uses καί-δέ-ἀλλά to cover the same "space" as English uses and-but, so δέ covers those cases in the middle where there is contrast but not quite opposition. One very common example of this is stuff like ὁ δὲ εἶπεν "and he said" where the δέ marks the contrast in that it's a new person speaking (more generally, a change in subject) but there's no sense of opposition.