

Мы долго говорили об этом вчера.
We talked about it for a long time yesterday.
Ты уже подумал об отпуске?
Have you thought about your vacation yet?
Я ничего не знаю об этой истории.
I don't know anything about this story.
"об" is in the lesson "Prepositions" (Level 36), along with 12 other useful words. LinDuo has 180 topic lessons like this one, 2,380 words in total. Every word has an illustration, a transcription and pronunciation in a female and a male voice. Just 10–15 minutes a day is enough to learn them all. Free, and works offline.