When simplifying algebraic expressions, Year 9 students often make some common mistakes. Here are a few important tips to keep in mind:

1. Combining Like Terms: Make sure to only add or subtract like terms. For example, $3x$ and $5x$ are like terms. When you combine them, you get $8x$.

2. Distributing Correctly: Use the distributive property the right way. If you see $2(a + 3)$, remember it becomes $2a + 6$. It does not turn into $2a + 3$.

3. Watch Out for Negative Signs: Be careful with negative signs! For example, in $-2(x - 4)$, you need to distribute the negative. This changes it to $-2x + 8$.

By keeping these points in mind, simplifying algebraic expressions will be much easier!