We can summarize the trailer for you if you missed it.
1) Carol grieves over the people she's killed.
2) Carol does her lone wolf schtick again during the Saviors raid that she feels obligated to do in order to keep her people safe except, this time, Maggie shows up unexpectedly which she had not accounted for.
3) This leads to the Saviors capturing both Carol and Maggie. We see the pained reaction on the faces of Glenn and Rick after they've evidently been successful in their initial frontal assault on a Saviors encampment.
EDIT: Or... you can scope out that YouTube link so long as it remains up.
Incidentally, I'm beginning to second-guess the intel that Tara gets killed this season. Think she may have been written out temporarily with Heath (as they go on their run) due to Alanna's pregnancy and that's why she wasn't around when key scenes were filmed along with everyone else.
I think Rosita takes the crossbow shot from Dwight that was reserved for Abe in the comics.
I remain firmly undecided between Glenn and Daryl on the Lucille issue.
I can see both Abe and Daryl stepping up as willing sacrifices during the finale only for Glenn to get his skull bashed in despite their efforts.