You can remember the meaning of 只能 with this easy mnemonic:
The parents were too busy to prepare for the sleep over so the kids only (只) had a jigsaw puzzle to play. They were so bored and could only finish half. Even if they were capable (能), there was actually a piece missing.
Using mnemonics to remember the meaning of Chinese characters like can have an enormous impact on how long to learn Mandarin it takes.