mjhjmtu

When studying I always come across words such as apartment, when I search many dictionaries I sometimes get an exact translation апартамент, but there is also another Russian word that has the exact same meaning, квартира.

What is the difference between them and what do I do during these cases?

It depends on the context.

Квартира is a flat where you live. One or several rooms, usually with a kitchen and a bathroom, etc., and this apartment is situated in a block of flats. Квартира в многоквартирном доме.

Апартамент, апартаменты is a kind of lodging to rent. We do not call our homes this word. Unless to make a phrase sound pompous or/and comical.

If you come to visit another country and search for lodging, you will be most likely offered апартаменты. And if you come to settle and make up your mind to purchase sth to live in, you will buy квартира.

According to the law, апартаменты is an apartment in a building not meant for permanent accomodation, but equipped as a flat. Usually it is well-equipped and large.

But commonly, an апартаменты you will be offered as a tourist can be a flat in a block, but with no permanent residents.

As Elena fully explained the difference between квартира and апартаменты, to answer the second part of the question:

what do I do during these cases?

In general one should be somewhat careful with obviously loaned words - they are usually loaned for a reason - to denote a special case for what a general word already exists.

For example consider a pair computer/компьютер - it should not come as a surprise that Russian already has an exact original analogue to the verb compute - вычислять, and a noun вычислитель is very much an exact translation of English computer. Yet, since electronic computers were invented in the West, that foreign-sounding word was adopted to specifically denote that novel and foreign technology. (As everything in language, it was not sanctioned by a central authority) It would be incorrect to use the word компьютер in relation to anything else, but electronic computers.

So when you have a few translation variants and one of them looks like a loaned word - most likely this variant denotes a special case, while there is a more general variant that covers way more meanings. In such situation a good modern dictionary is very nice to have. Another option would be to google the use of each variant in Russian texts to get an idea how they differ in usage. Event a simple search count can give you an idea of how widely this or that variant is used.

Квартира is the word to be used in most cases.

Аппартаменты (usually PLURAL) as slightly archaic is used to denote reception rooms in palaces, luxury suites, etc. So it's usually not used for modern flats in modern homes. But could be used in hotel room descriptions and ads, often luxury ones.

But it must be mentioned that this term is CURRENTLY becoming popular for some new kind of real estate in modern market. The previous answers and comments were mentioning some law, but in fact THERE IS NO HOUSING LAW where this term is described (housing laws describe only the term КВАРТИРА, but not АППАРТАМЕНТЫ), so it's legally as it was mentioned here is regarded as commercial property (partly, because of that LACK OF DESCRIPTION IN THE LAW). The only legal document I've found is classification of tourist accomodations (where аппартамент as type of accomodation is used, but only in hotel/hospitality contexts).

So it's good to use квартира. Since аппартаменты in this new sense is not quite described it's better to avoid using it in modern contexts (it's just a new DEVELOPPERS' term I would say). But аппартаменты sounds good in literature texts where palaces, luxury rooms are described.