Diagnosta wyjaśnił następny krok.

Because some Polish masculine nouns, especially professions or roles, end in -a. Diagnosta is one of them. Even though it looks a bit like a feminine noun, in this sentence it is masculine, and the verb shows that with wyjaśnił.

A feminine form would usually be diagnostka, and then the verb would be wyjaśniła.

It is in the nominative singular, because it is the subject of the sentence: the person who performed the action.

So the basic structure is:

Diagnosta — subject
wyjaśnił — verb
następny krok — direct object

Wyjaśnił is the past tense, 3rd person singular, masculine form of the verb wyjaśnić, meaning to explain.

So it tells you that:

• the action happened in the past,
• the subject is he or a masculine noun,
• the action is viewed as completed.

Related forms:

• wyjaśniła — she explained
• wyjaśniło — it explained
• wyjaśnili — they explained (masculine personal)
• wyjaśniły — they explained (non-masculine-personal)

This is a question of aspect.

• wyjaśnić → wyjaśnił = perfective, focusing on a completed action
• wyjaśniać → wyjaśniał = imperfective, focusing on the process, repetition, or ongoing action

So:

• Diagnosta wyjaśnił następny krok means he explained it, as a completed event.
• Diagnosta wyjaśniał następny krok would suggest he was explaining it, or spent time explaining it.

In a simple statement about one finished action, wyjaśnił is the natural choice.

It is the accusative singular, because it is the direct object of wyjaśnił.

The verb wyjaśnić normally takes a direct object in the accusative.

Because krok is a masculine inanimate noun, and in the singular, masculine inanimate nouns usually have the same form in the accusative as in the nominative.

So:

• nominative: następny krok
• accusative: następny krok

That is why nothing visibly changes here, even though the phrase is functioning as the object.

Yes, very natural. Następny krok is a common and normal phrase for the next step in a process, plan, instruction, or procedure.

A similar word is kolejny, which can also mean next or another following one, but następny krok is completely standard and idiomatic.

Yes. Polish word order is more flexible than English because case endings show the grammatical roles.

The neutral order here is:

Diagnosta wyjaśnił następny krok.

But you could also say:

• Następny krok wyjaśnił diagnosta.
• Wyjaśnił następny krok diagnosta.

These alternatives are possible, but they change the emphasis or sound more marked. The original sentence is the most neutral and straightforward.

Because Polish has no articles. There is no direct equivalent of English a/an or the in ordinary usage.

So Diagnosta wyjaśnił następny krok could correspond to:

• The diagnostician explained the next step
• A diagnostician explained the next step

Which one is meant depends on context.

Yes, often it could. Polish frequently drops subject pronouns and sometimes even noun subjects if they are already clear from context.

So if everyone already knows who is being talked about, you could say:

Wyjaśnił następny krok.

The verb wyjaśnił already tells you the subject is masculine singular. Still, keeping diagnosta makes the sentence clearer and more explicit.

A rough English-friendly approximation is vuh-YASH-neew, though that is not exact.

A few useful sound notes:

• w sounds like English v
• y is a Polish vowel with no exact English equivalent
• ś is a soft sh-like sound
• ń is a soft n
• ł sounds like English w

So the whole sentence is roughly:

dya-GNOS-ta vuh-YASH-neew NAS-temp-nih krok

The stress is usually on the second-to-last syllable in Polish words:

• diaGNOSta
• wyJAŚnił
• nasTĘPny