When we just name people (or things) as being (strong, intelligent, stupid, heavy, beautiful...) or as doing (running, working, laughing...) we use the nominative form.

When we say what happens to people (or things), we use a different form. English has different forms as well, but only for pronouns:

nominative	verb	accusative
I	see	him
he	sees	me
we	praise	her
he	accuses	them
they	admire	us

 
The name accusative comes from the Latin word accusare meaning, surprise, surprise, to accuse. It should by now be obvious to all that these names: nominative, accusative... are of relatively recent coinage. The Greeks used these forms long before coming into contact with the Romans and their Latin language. So they had different words for them, Greek ones. As they did for all other grammatical terms. But we won't bother with those just yet.
The terms themselves are useful for naming the different forms taken by words according to usage. And they are easy to remember because they combine name and usage in one word:

Instead of accuse we could say: see, hear, love, hate, hit or adore. This form could have been called anything, but accusative it became! And still is.

Now in Greek (like in Latin, German, Russian...) all articles, nouns, pronouns and adjectives have special forms according to meaning and context.
The form called accusative for -ος words is -ον , in other words: ς changes to ν. Not too difficult.
So: When in English you would say "him", in Greek you use -ν at the end of the word.

ὁ Παῦλος λύει τὸν Πέτρον (he frees him) is not the same as:
τὸν Παῦλον λύει ὁ Πέτρος (him he frees). In the first sentence Paul is named as letting go, in the second Peter.

Word order in Greek is much less rigid than in English, forms will always tell who does what.

ὁ ἰσχυρὸς Παῦλος τὸν μικρὸν Πέτρον φέρει.
is the same as:
τὸν μικρὸν Πέτρον ὁ ἰσχυρὸς Παῦλος φέρει
but
τὸν ἰσχυρὸν Παῦλον ὁ μικρὸς Πέτρος φέρει
is quite a different proposition altogether. How would the small man manage to carry the strong one? Maybe he is Asterix (who, as a baby, fell into the magical potion prepared by the Druid and as a result is the strongest little man anybody has ever come across).

Watch out: Article and adjective and noun that belong together will therefore all change together according to meaning.

Now use the form! Theory is all very nice, but practice makes perfect. So, with your mind's eye see people or things doing.... and give voice to your thoughts. You don't like my "things" doing? So what about: Steak and chips give me great pleasure? That's using παρέχει, unfortunately we haven't had either "steak and chips" nor "pleasure", so have to use the English words. Which of course doesn't allow us to practice nominative and accusative form, only the verb "to give, provide". A great shame. Better think of something else.