七年级下册英语肯定句的基本结构为：
主语 + be动词/助动词/情态动词 + 动词原形
例如：
1. I am reading a book.
2. She can swim.
3. They like playing football.
一、一般现在时肯定句
be动词（am/is/are）+动词原形 I am a student.
He is a teacher.
They are my friends.
情态动词+动词原形
You can speak English.
It must be sunny today.
She should finish her homework.
二、一般过去时肯定句
be动词过去式（was/were）+动词过去式
He was born in 1990.
We watched a movie last night.
They visited their grandparents last summer.
助动词did+动词原形
She did her homework on time.
He did not （didn't） call you last week.
We did not （didn't） understand the problem.
三、现在进行时肯定句
be动词（am/is/are）+动词ing形式
I am studying English now.
She is painting a picture.
They are playing soccer on the field.
四、特殊疑问句（需转换为陈述句）
特殊疑问句需通过交换主语和be动词来转换为陈述句，例如：
特殊疑问句： What does he do？ 转换后
五、其他常见结构
情态动词+动词原形（表示能力、许可等）
You may leave early.
The cat can climb trees.
不定式短语作后置定语
The book that I borrowed is very interesting.
以上结构覆盖了七年级下册英语中常见的肯定句类型，建议结合具体语境进行练习。