also也用于肯定句，常用于句子中间，放在be动词、助动词或情态动词之后，行为动词之前.
e.g. He also swims fast. 他也游得很快.
The girl can also ride a bike. 那个女孩也会骑车.
only用作形容词,在句中作定语。
1. 与the连用,意为"唯一的".如:
John is the only boy in his family. 约翰是他家里唯一的男孩。
She is the only girl here who knows how to drive a car. 她是这儿唯一会开车的姑娘。
2. only可修饰复数名词,意为"仅有的;唯一的"。如:
They were the only people present. 出席的只有他们这些人。
3. 与an连用,意为"单独的"。如:
Jim was an only son of his parents. 吉姆是他父母的独子。
4. 与the连用,意为"最好的;最适当的"。如:
She is the only person for the job. 她是做这项工作的最佳人选。
The only thing to do on a hot day is to go swimming. 在热天最理想的事就是去游泳