Distributive X po X

English has a handy way to say that something happens gradually, one piece at a time: step by step, word by word, day by day. Serbian does exactly the same thing — it repeats the noun and puts po between the two copies: korak po korak, reč po reč, dan po dan.

The whole expression works like an adverb: it tells you how something is done, so it can stand at the start, in the middle or at the end of the sentence, and it never changes with the subject or the tense.

Which form do the nouns take?

Here is the one thing to get right. Both nouns are the same form, and that form is the accusative singular — the form you already use for a direct object. For most nouns this is simply the dictionary form, so nothing changes at all. Only feminine nouns ending in -a show it: their -a becomes -u, and it becomes -u on both copies.

Do not mix forms. Say stranicu po stranicu, never stranica po stranicu or stranicu po stranici. Elsewhere po is often followed by the locative (po gradu — around town), but in this pattern it is not: the second noun simply copies the first.

What po means here

In this pattern po means one after another, unit by unit. It does not mean around or across as in šetamo po gradu. Think of it as the by in step by step. The noun names the unit of progress: a step, a word, a day, a room, a stone.

• Učim srpski korak po korak. — I'm learning Serbian step by step.
• Dete čita slovo po slovo. — The child reads letter by letter.
• Prevodila je tekst reč po reč. — She translated the text word by word.
• Radnici zidaju kuću ciglu po ciglu. — The workers are building the house brick by brick.
• Voda curi kap po kap. — Water is dripping drop by drop.

Two close relatives you will meet: malo po malo (little by little) and jedan po jedan (one by one). They follow the same idea of repeating a word around po.