Here are the mistakes in each sentence:

1. **Mistake:** "the populate"
**Correction:** "the population"
**Corrected sentence:** "The population of the world has increased in modern times."

2. **Mistake:** "not enough old"
**Correction:** "not old enough"
**Corrected sentence:** "He is not old enough to join the army."

3. **Mistake:** "can they?"
**Correction:** "can they?" is correct, but the statement should have a tag question in the form of "can't they?" since "everyone" is a singular noun.
**Corrected sentence:** "Everyone can learn how to swim, can't they?"

4. **Mistake:** "since three years"
**Correction:** "for three years"
**Corrected sentence:** "She has learnt English for three years, hasn't she?"

5. **Mistake:** "the price of the houses have"
**Correction:** "the price of the houses has" (since "price" is singular)
**Corrected sentence:** "The price of the houses has doubled over the past few years."

Let me know if you need further clarification!