

Сегодня был сложный день.
Today was a difficult day.
Это очень сложный вопрос.
That's a very difficult question.
Тест был не очень сложный.
The test wasn't very difficult.
"сложный" is in the lesson "Adjectives 2" (Level 4), along with 17 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.