Original sentence: Mountain plants grow in tight clumps. This helps them hold onto steep slopes and lose less water.
Is it correct? Yes — the sentence is grammatically correct. It makes sense and the words are in the right order.
Small improvement (one sentence):
Mountain plants grow in tight clumps, which helps them hold onto steep slopes and lose less water.
Why this is a little clearer:
- Putting the two ideas into one sentence with which shows that the second part explains the first part.
- "Tight clumps" means the plants grow close together in small groups.
- "Hold onto steep slopes" means the plants can stay on the steep hills and not fall away.
- "Lose less water" means the plants keep more water, so they don’t dry out as fast.
Even simpler version for a 7-year-old:
Mountain plants grow close together. This helps them stay on steep hills and keep more water.
All versions are fine; the change is just to make the connection between the two ideas a bit clearer.