He said, "let's go"
(convert into indirect speech)

This is a "suggestion". Whenever the word "Let" is used in a sentence, It's a suggestion. This is a type of imperative sentence.

So, it would be like:

=> He suggested to go.

Explanation:

When the exact words used by a speaker are reproduced, it is called direct speech. When the meaning of a speech is conveyed in the reporter's word, it is called Indirect speech.

General Rules (Applied in each sentence)

For eg:

John says," I go home".

= John says that he goes home

Simple present -----> simple past

Present continuous ---> past continuous

Present perfect -----> past perfect

Present perfect continuous--> past perfect

continuous

Simple past --->past perfect

Past continuous--> past perfect continuous

Example:

Sophia said, " I went home".

= Sophia said that she had gone home.

Example:

Olivia said to me, " You speak English"

= Olivia told me that I spoke English.

Note: Third person of reported speech is never changed.

Example:

Anna said to you , " I lost my pen".

= Anna said to you that she had lost her pen.

If 'we' is used in reported speech, instead of human beings or mankind, at that time 'we' is not changed.

Hope this helps...

Good luck on your assignment...

grunge