Matthew 1:20

Ταύτα δε αυτού ενθυμηθέντος
ιδού άγγελος κυρίου
κατ' όναρ εφάνη αυτῴ
λέγων,
Ιωσήφ υιός Δαυίδ,
μη φοβηθῄς παραλαβείν Μαρίαμ την γυναίκα σου,
το γαρ εν αυτῄ
γεννηθέν εκ πνεύματός εστιν αγίου

Now here's a verse to sink your teeth into. Let's start with a mostly word-for-word translation of the first three lines above:

Ταύτα δε αυτού ενθυμηθέντος
these things and/but/_ he considered

ιδού άγγελος κυρίου
look angel of lord

κατ' όναρ εφάνη αυτῴ
in a dream appeared to him

Once we've remembered to move 'δε' to the first position, i.e.

And/but these things he considered

the first line makes more sense. This line will also make more sense when we recognize that it is another example of a genitive absolute, that is, a noun or pronoun (here, αυτόυ, 'he') plus an accompanying participle(ενθυμηθέντος), both in the genitive.

The participle is an aorist form of the deponent verb 'ενθυμέομαι', meaning something like 'deliberate, consider, ponder'. Because it is in the aorist, it gives the sense that the deliberating, considering, or pondering has been completed, so we could add a word in the English translation to point to that sense, e.g.:

And after he pondered these things

or

And when he had considered these things

Who had pondered?: 'αυτού', 'he', i.e., Joseph. Remember that although we normally place the subject of a verb in the nominative case, this is a genitive absolute, and the word 'he' ('αυτού) is in the genitive. Do not translate it 'of him'.

So after Joseph had pondered these things (i.e., his betrothed Mary had been found to be pregnant, and he had resolved to send her away quietly) what happened next?

ιδού άγγελος κυρίου
look angel of lord in a dream

κατ' όναρ εφάνη αυτῴ
in a dream appeared to him

The word 'ιδού' appears many times (200) in the New Testament, and is often translated 'behold', although in some versions (the New Revised Standard and the New International, for example) it is left untranslated. For more information about this word, left-click on it at greekbiblestudy.org.

An angel of the lord ('άγγελος κυρίου') is the subject here, and 'εφάνη' ('appeared') is the verb. Note that we have to add the word 'the' to make this phrase idiomatic English: i.e., 'an angel of the lord', not 'an angel of lord'.

So the angel appeared 'to him' (αυτῴ - the third person, singular, masculine pronoun in the dative) -- i.e., appeared to Joseph.

Only one thing is left: An angel of the lord appeared to Joseph - how? The phrase 'κατ' όναρ' means 'in a dream.'

So an angel of the lord appeared to Joseph in a dream. Next time we will continued with the rest of the verse.

Χαίρετε!