Ovaj plan je okej.

Ovaj means this and it agrees with plan.

In this sentence, ovaj is a demonstrative word pointing to a specific plan: this plan.

It is in the:

• masculine
• singular
• nominative

That is because plan is a masculine singular noun and it is the subject of the sentence.

So:

• ovaj plan = this plan

If the noun were feminine or neuter, the form would change:

• ova ideja = this idea
• ovo rešenje = this solution

In Serbian, every noun has a grammatical gender: masculine, feminine, or neuter.

Plan is a masculine noun. One common clue is that many masculine nouns in the dictionary form end in a consonant, and plan does.

Here it appears in the nominative singular, which is the basic form used for the subject:

• plan = plan

Because plan is masculine singular, the demonstrative has to match it:

• ovaj plan not
• ova plan not
• ovo plan

Je is the 3rd person singular present tense form of the verb biti = to be.

So it means is.

In this sentence:

• Ovaj plan je okej.
• This plan is okay.

Serbian usually uses this verb in sentences like this, just as English does.

A very important detail: je is a clitic, which means it normally does not stand in the first position of the sentence and tends to come early, often in the second position.

This is the most neutral, natural word order.

A key reason is that je is a clitic, and clitics in Serbian usually come near the beginning of the sentence, often in second position.

So:

• Ovaj plan je okej. is very natural.

You may also hear other orders in speech for emphasis, but they sound marked or stylistically different:

• Okej je ovaj plan. = more emphasis on okay
• Ovaj je plan okej. = possible, but different focus

For a learner, Ovaj plan je okej is the safest standard pattern.

Yes. Okej is very common in modern Serbian, especially in everyday speech and informal writing.

It is basically a borrowed word from English okay/OK, written in a Serbian-style spelling:

• okej

It is often used to mean:

• okay
• fine
• acceptable
• all right

It sounds informal to neutral. Depending on context, more native-sounding alternatives could be:

• dobar = good
• u redu = okay / in order
• prihvatljiv = acceptable

So Ovaj plan je okej sounds natural, but a bit conversational.

Usually, okej is treated as an indeclinable word in everyday use, especially in sentences like this.

So you can say:

• Plan je okej.
• Ideja je okej.
• Rešenje je okej.

The form okej stays the same.

That is different from a regular adjective like dobar, which changes:

• Ovaj plan je dobar.
• Ova ideja je dobra.
• Ovo rešenje je dobro.

So one reason learners like okej is that it is simple and does not change.

A useful approximate pronunciation for an English speaker is:

OH-vai plan yeh oh-KAY

A few notes:

• j in Serbian is pronounced like English y in yes
• je sounds like yeh
• ovaj ends with that -aj sound, roughly like eye
• okej sounds very close to okay, but with Serbian spelling

So:

• ovaj ≈ OH-vai
• je ≈ yeh
• okej ≈ oh-KAY

Yes, absolutely.

• Plan je okej. = The plan is okay / A plan is okay, depending on context
• Ovaj plan je okej. = This plan is okay

Adding ovaj makes it specific and points to a particular plan.

So if you are comparing several plans, ovaj helps make it clear:

• Ovaj plan je okej, ali onaj nije.
• This plan is okay, but that one isn’t.

You could, but it does not mean exactly the same thing.

• Ovaj plan je okej. = This plan is okay.
• Ovo je okej plan. = This is an okay plan.

The second sentence uses ovo je = this is, and okej describes plan more directly.

So the structures are different:

• ovaj plan = this plan
• ovo je = this is

For the sentence you were given, Ovaj plan je okej is the correct match.

It is mostly neutral to informal because of okej.

The grammar itself is completely normal, but okej sounds conversational. In more formal contexts, people might prefer:

• Ovaj plan je dobar.
• Ovaj plan je u redu.
• Ovaj plan je prihvatljiv.

So:

• Ovaj plan je okej. = natural in everyday conversation
• less likely in very formal writing

Normally, no. In a standard present-tense sentence like this, you should keep je.

So:

• Ovaj plan je okej. = correct
• Ovaj plan okej. = not standard Serbian

Serbian generally requires the present-tense form of to be in this kind of sentence.

That makes it closer to English here:

• This plan is okay not
• This plan okay

You replace je with nije:

• Ovaj plan nije okej.
• This plan is not okay.

So:

• je = is
• nije = is not

This is a very common pattern in Serbian:

• To je dobro. = That is good.
• To nije dobro. = That is not good.