If you study Russian, you probably already know that there are two ways of addressing a person, informal “ты” and a form of courtesy “вы”. This concept doesn’t exist in modern English, that conserved only “you” form, but can be met in other European languages, like French, German or Spanish.

When to use “вы”

The courtesy form “вы” coincides with the 2nd person plural, so when you use this form to address one person, you also use the same plural form for verbs. E.g. Вы мне не поможете? (Could you help me?). The idea came to Russian mainly through French, so if you know this language, you may see some similarities. “Вы” is a widely used form in Russian, and if you are not sure which form to choose, it’s always safe to choose “вы”. Here some situations for you, when you certainly should choose “вы”:

To address a stranger. It’s always a good idea to use “вы” to address someone you don’t know regardless their age. A stranger on the street, a waiter at a café, a shop assistant, a Russian client who calls to your office. It’s not typical for Russians to use “ты” to address strangers and in this context “ты” may sound quite rude.
When you address someone older than you. Regardless the context, if you address someone who is (much) older than you, and who is not your family member, it is appropriate to use “вы”.
If you address your teacher. In Russian learning institutions, like school or university, you should use “вы” form to address your teacher, unless the teacher asks to do otherwise.
At work. In formal situations, Russians prefer “вы” form. Therefore, it’s absolutely regular thing to address your clients, partners and coworkers this way.

When to use “ты”

“Ты” is the regular 2nd person singular form. It is mostly used in informal situations and shows a certain kind of proximity between people. It’s correct to use “ты” in the following situations:

With friends and family
In informal context with people of your age or with people, you already know well. However, if it’s someone much older than you, it’s better to ask before start using “ты”, even if you know the person quite well.
When you address children. Russians normally don’t use “вы” to address children, so using “ты” in this case is absolutely fine.

A transition from “вы” to “ты”

Once the context gets less formal and you get to know a person, it’s natural to move from “вы” to “ты”. There is no solid rule on when this transition happens; most likely, your Russian acquaintance will let you know when it’s fine to move to “ты”. It may happen even in some formal situations, for example, in many companies, colleagues use “ты” to address each other, and especially it is typical for younger generations.