We don't need to change the tense, though probably we do need to change the 'person' from 'I' to 'she', for example. We also may need to change words like 'my' and 'your'.
But, if the reporting verb is in the past tense, then usually we change the tenses in the reported speech:
Direct speech: I like ice cream.
Reported speech: She said she liked ice cream.
I like ice cream
She said (that) she liked ice cream.
I am living in London
She said she was living in London.
I bought a car
She said she had bought a car OR
She said she bought a car.
I was walking along the street
She said she had been walking along the street.
I haven't seen Julie
She said she hadn't seen Julie.
I had taken English lessons before
She said she had taken English lessons before.
I'll see you later
She said she would see me later.
I would help, but..
She said she would help but...
I can speak perfect English
She said she could speak perfect English.
I could swim when I was four
She said she could swim when she was four.
I shall come later
She said she would come later.
I should call my mother
She said she should call her mother
"I might be late"
She said she might be late
"I must study at the weekend"
She said she must study at the weekend OR She said she had to study at the weekend
* doesn't change.
Occasionally, we don't need to change the present tense into the past if the information in direct speech is still true (but this is only for things which are general facts, and even then usually we like to change the tense):
Direct speech: The sky is blue.
Reported speech: She said that the sky is/was blue.
So now you have no problem with making reported speech from positive and negative sentences. But how about questions?
Direct speech: "Where do you live?"
How can we make the reported speech here?
In fact, it's not so different from reported statements. The tense changes are the same, and we keep the question word. The very important thing though is that, once we tell the question to someone else, it isn't a question any more. So we need to change the grammar to a normal positive sentence. Confusing? Sorry, maybe this example will help:
Direct speech: "Where do you live?"
Reported speech: She asked me where I lived.
Do you see how I made it? The direct question is in the present simple tense. We make a present simple question with 'do' or 'does' so I need to take that away. Then I need to change the verb to the past simple.
Direct speech: "where is Julie?"
Reported speech: She asked me where Julie was.
The direct question is the present simple of 'be'. We make the question form of the present simple of be by inverting (changing the position of)the subject and verb. So, we need to change them back before putting the verb into the past simple.
Here are some more examples:
Where is the Post Office, please?
She asked me where the Post Office was.
What are you doing?
She asked me what I was doing.
Who was that fantastic man?
She asked me who that fantastic man had been.
So much for 'wh' questions. But, what if you need to report a 'yes / no' question? We don't have any question words to help us. Instead, we use 'if':
Direct speech: "Do you like chocolate?"
Reported speech: She asked me if I liked chocolate.
Sometimes when we change direct speech into reported speech we have to change time expressions too. We don't always have to do this, however. It depends on when we heard the direct speech and when we say the reported speech.
It's Monday. Julie says "I'm leaving today".
If I tell someone on Monday, I say "Julie said she was leaving today".
If I tell someone on Tuesday, I say "Julie said she was leaving yesterday".
If I tell someone on Wednesday, I say "Julie said she was leaving on Monday".
If I tell someone a month later, I say "Julie said she was leaving that day".
So, there's no easy conversion. You really have to think about when the direct speech was said.
Here's a table of some possible conversions:
then / at that time
yesterday / that day / Tuesday / the 27th of June
the day before yesterday / the day before / Wednesday / the 5th of December