This is the specific feature of the Russian language – nouns do change their endings. But they do not do it randomly: their endings change according to the grammatical requirements of a sentence.
In our course we start with the Prepositional case (4.7). The most common prepositions here are:
в - in/at
на - on/at
о – about
If you compare how words appear in a dictionary (or their nominative case) with how they change when the above prepostions are used before them – you’ll get an idea:
парк (park) в парке (in the park) в парках (in parks)
школа (school) в школе (at school) в школах (at schools)
станция (station) на станции (at the station) на станциях (at stations)
стол (table) на столе (on the table) на столах (on tables)
вино (wine) о вине (about wine) о винах (about wines)
and you can continue the list in the same manner using the examples from our lessons. You can try different prepositions with the same word., like:
город – в городе – в городах
о городе – о городах
Soon, you'll start to see the pattern.
There are some exceptions, of course, they are best to be memorised as a whole phrase, such as.: в шкафу (in the cupboard)
Once you feel comfortable with this case, you can move on to the next one. But make sure, you have plenty of practice with it. Just take one case at a time, otherwise it may seem overwhelming.