only that endings change. for example ja ide do sklepu...ale jestem w sklepie
it is true in majority of cases - but I even hesitate to say in most cases
see for example 'pies'
'To jest pies' - This is a dog. (nominative)
'Złapałem psa' (and not 'piesa') (though one can say 'piesa' for the sake of being humorous ;) - I caught a dog. (accusative case - because łapać/złapać 'demands' accusative - some other verbs demand different cases
'Złapałem w końcu tego psa' - I have caught that/the dog at last. - intonation in that sentence makes a difference in translation (that or the).
'Szukam niedużego psa' - I look for a little (smallish?) dog (nieduży is sort of not big in a 'literal' translation - but 'nieduży' is rather bigger than 'tiny') - genetive here - for living things of masculine gender (grammatical masculine gender) accusative form is the same as genetive - szukać requires genetive
Dajmy temu psu pić. - Let's give that dog some water. (dać pić means to give something to drink actually but it would look strange in English)
psu is a dative form (dative in Latin is related to giving)
Pogoda pod psem. (literally Weather under a dog - it means bad weather) - psem is a instrumental form - pod can go with two different cases but then it has different meanings for either - when movement is involved (like when you want to say you want to put something under a dog) accusative is required - otherwise it's instrumental.
'Ten koc śmierdzi brudnym psem' - This blanket stinks of a dirty dog. - some verbs that do require a preposition in English don't require one in Polish (and vice versa). - look at these two sentences 'Pies śmierdzi'. 'Psem śmierdzi.' (A dog stinks. It stinks of a dog.) 'Psem' is not the subject but the object. There is no explicit subject in 'Psem śmierdzi' The subject is in nominative in most cases.
Na spacer z psem. - For a walk with a dog. with (together with) gives z + instrumenatal in Polish
W tym psie znaleziono dwie kule myśliwskie. Two hunting weapon bullets were found in this dog. - it's locative here - the bullets found sitting in the unfortunate dog - (w preposition is similar to pod in accepting two cases - for situations where movement is observed it is accusative again and locative for 'static' situations: Strzelił w psa. He shoot at a dog. (one can say that this w is more like into than in) )
The remaining case is vocative - it's used as a direct address like in God, help me. - Boże, pomóż mi. It's not like all direct addresses in Polish make use of vocative. When you turn to people with their first name in casual speech, the name is in nominative. (Marek, do domu. Mark, come home).
Vocative tends to be used when one wants to show affection. But it is also used to show dissatisfaction, disaproval. In case of 'pies' (animals but also things) if someone wants to show affection one makes a diminutive of it - a diminutive of 'pies' is 'piesek' and the final address is 'piesku'. (Nie bój się, piesku - Don't be affraid doggy.) The vocative of 'pies' is 'psie' and it tends to carry negative emotional load and is rarely used in neutral contexts. (Ty cholerny głupi psie. You bloody stupid dog.) Notice that vocative and locative of 'pies' are the same. It's the same with all nouns of masculine gender (but not it's not the case with other genders)
Consider yourself lectured ;)
To repeat the point of this lecture - it's sometimes not only the endings that change but some vowels can be 'swallowed' to create declination cases (pies - psa - psu - psem - psie) (To augment it the plural of 'pies' is 'psy' ;)
'Koniec' 'An/the end' is similar to 'pies' (koniec - końca - końcu - końcem: 'e' gets swallowed 'i' transforms into the 'accent' this time (the accent denotes palatal variety of a consonant)