2. The boy isn't old enough to understand the problem.
3. My drawing is good enough to win the competition.
4. This novel is interesting for us to read it.
5. The water is warm enough for me to drink it.
Structure:
$S_1$+be+(not)+adj...$S_2$+V+...
→ $S_1$+be+(not)+adj+enough+(for $S_2$)+to V...