Matthew 1:21 - finishing up

τέξεται δε υιόν και καλέσεις το όνομα αυτού Ιησούν,
αυτός γαρ σώσει τον λαόν αυτού από των αμαρτιών αυτών

For she will bear a son, and you shall call his name Jesus,
for he will save his people from their sins.

We've already seen that the verb 'τέξεται' has Mary as the understood subject; it is Mary who will bear a son ('υιόν'). But the next verb has 'you' as the understood subject; 'καλέσεις' is the second personal singular of the future of the verb 'καλέω', 'I call'.

The 'you' refers, of course, to Joseph, who is being directly addressed (in a dream) by an angel. So Joseph will call . . . what?

. . . . καλέσεις το όνομα αυτού Ιησούν
. . . . you will call the name of him Jesus

Or, in more idiomatic English,

you will call his name Jesus

or even (slightly simplified, but even more idiomatic):

you will call him/name him Jesus

The next line also has a verb in the future: 'σώσει' - he/she/it will save

In this case, we read 'he will save', not only because the verse is clearly referring to Jesus, but also because we have an added pronoun, 'αυτός' - 'he'. The use of the pronoun 'αυτός' is a complicated bit of Greek; for now we will note that in this context it adds emphasis.

'He will save'.

Exactly what/how/who will Jesus be saving?

τον λαόν αυτού από των αμαρτιών αυτών
his people from their sins

The direct object of 'σώσει' is 'τον λαόν' - 'the people'. Followed by the third person pronoun in the genitive ('αυτού')- indicating possession - it becomes 'his people'.

Then we finish the verse with a prepositional phrase:

από των αμαρτιών αυτών
from the sins of them

The preposition 'από' takes the genitive; this accounts for the word for 'sin' ('η αμαρτία') being in the genitive (plural). The final word is once again the third person pronoun in the genitive, but this time it is the genitive plural - 'αυτών' - 'their'.

That's it for the verse. Χαίρετε!