Substitution n. 取代反应

Substitution involves replacing an atom by another atom .
取代反应就是由一个原子取代另一分子中的一个原子。

Addition reaction n. 加成反应

Addition reactions involve two molecules joining together to form a single new molecule. 
加成反应表示两个分子结合生成一个分子的过程。

Elimination n. 消去反应

Elimination involves the removal   of a molecule from a large one. The addition of ethene to steam may be reversed by passing ethonal vapour over a hot catalyst such as pumic. A water molecule is eliminated.
CH3CH2OH(g) → CH2=CH2(g) + H2O(g)  
消去反应表示的是从一个大分子中移走一个小分子的过程。例如在乙醇中加入催化剂可以从乙醇中移走水分子。

Hydrolysis n. 水解反应

Hydrolysis reactions involve breaking covalent bonds by reaction with water. The substitution of the bromine atom in bromoethane by hydrolysis is also a hydrolysis. The reaction proceeds much more slowly in water.
CH3CH2Br(l) + H2O(l) → CH3CH2OH(aq) + HBr(aq)    
水解反应是指和水反应断开一个共价键的过程。   例如溴乙烷和水反应生成乙醇和溴化氢的过程。
这几种反应是考试必考内容，写反应方程式，要注意反应条件啊。