途中まで見たよって英語でなんて言うの？

I've only seen a few episodes so far. - a few means not many. You would use this phrase if you haven't seen a lot of the season.

I started the season, but I haven't finished yet. - you can also express that you're only partially through by saying that you started but haven't finished.

A drama series is made up of many episodes with a continuing story or plotline.
1. I haven't watched the entire series.
This means that you have not watched all the episodes to complete watching the entire series.

2. I've only seen some episodes.
   This is a simple and direct statement. If you have only seen some episodes it means that you have not watched the entire series.

To tell someone about a TV show or movie that you have watched a part or half of, however you haven't seen the ending yet, you could say it like this... "I watched half of/part of it, but not the ending."

You may explain exactly how many episodes you have already watched - or explain up to which episode you have completed viewing:
"I'm up to series 2, episode 3."
Or you could talk about the portion of the entire series that you have already watched:
"I've seen about fifty percent of this series so far"

▪ I am halfway through the series.
This is indicating that you only watched 50% of the series.

▪ I've only watched half of the 1st season.
This is indicating that you did not finish the series and only watched a section of it.

Example 1
Friend: Did you finish the new series?
You: I am halfway through the series.

Example 2
Friend: How far did you watch the new series?
You: I've only watched half of the 1st season.

The finale is the way something ends...the final act as it were.
When we discuss any TV series with friends... there is a danger of "SPOILERS"
major plot elements may be revealed before we have seen them unfold!
I like to wait till I have seen the finale before discussing shows;-D
"I have not reached the finale yet, I have only watched some of the episodes"

These sentences all describe that said person has not watched the whole series on T.V only half of it.