Questions & Answers about nǐ gǎnmào le, yào duō hē shuǐ.
In 你感冒了, 了 does not simply mean past tense.
Here, 了 marks a new situation or change of state:
- before: you were fine
- now: you have a cold
So 你感冒了 means something like:
- You’ve caught a cold
- You have a cold now
Without 了, 你感冒 sounds less natural as a normal statement about your current condition.