If someone is interested, he adds his name to the list.
If someone is interested, they can add their name to the list.
If you are interested, then you can add your name to the list.
Someone had parked his car in the middle of the street.
Someone had parked their car in the middle of the street.
Someone called for you this morning, but he didn’t leave his name.
Someone called for you this morning, but they didn’t leave their name.
A man / woman called for you this morning but he / she (they) didn’t leave his / her (their) name.
(On the phone) Who is speaking? I am Anna.
(On the phone) Who is speaking? It’s me (Anna). / This is Anna.
The job is not as easy as someone might expect.
The job is not as easy as you / one might expect.
With this app, the user can do x and he can also do y.
With this app, users can do x and they can also do y.
With this app, you can do x and you can also do y.
If someone has a commercial activity, they will suffer in an economic crisis.
If you have a commercial activity, you will suffer in an economic crisis.
People who have a commercial activity will suffer in an economic crisis. They will find it hard.
Someone rang, but they didn’t leave their name.
A man (woman) rang, but he (she) didn’t leave his (her) name.
The user has to insert their (his/her) password before accessing the site. = Users have to insert their ...
If you have a lawyer, then you can ask them for an opinion.
She has just had a baby. Is it a boy or a girl? It’s a girl.
One never knows what one’s future will hold, does one? (very formal, pompous, archaic)
You never know what your future will hold, do you? (normal form)
he (him, his) should never be used as a generic pronoun, e.g. to refer to a generic lawyer, engineer, doctor etc. These are all jobs that can be done by women too. So use they, them, their.
When someone (meaning one particular person) is the subject of the sentence, use they as the pronoun; however, if someone refers to a generic person (i.e. everyone in such a situation), then prefer you.
When asking the sex of a child or an animal use it. Also, use it to refer to someone unseen: I can hear someone in the next room, I wonder who it is.
When announcing who you are on the phone, use this or it: This is John speaking. It’s John here.
The impersonal pronoun one is very formal, prefer you.
In written documents, avoid using he / she by making the subject plural and using they. In any case, even with a singular subject (e.g. the user), you can still use the plural pronoun (they, them, their).