Ne slažemo se sa planom.

Slažemo se means we agree.

The basic verb is slagati se, which means to agree. In this sentence it appears in the 1st person plural present tense:

• slažem se = I agree
• slažeš se = you agree
• slaže se = he/she/it agrees
• slažemo se = we agree
• slažete se = you all / you formal agree
• slažu se = they agree

So Ne slažemo se = we do not agree.

Se is a reflexive particle that belongs to the verb slagati se.

In Serbian, some verbs regularly appear with se, and you usually need to learn them that way as one unit. So it is best to treat slagati se as the full dictionary form meaning to agree.

Without se, slagati can have other meanings, so se is important here.

Ne is the negation word, meaning not.

In Serbian, ne normally goes directly before the verb:

• Slažemo se. = We agree.
• Ne slažemo se. = We do not agree.

So the negative is formed very simply by adding ne before the verb form.

Because the verb slagati se is followed by sa + instrumental case when you mean to agree with someone or something.

So:

• sa planom = with the plan
• sa njim = with him
• sa tobom = with you

The noun plan changes to planom because it is in the instrumental singular.

The noun plan is a masculine noun. In the instrumental singular, many masculine nouns take the ending -om.

So:

• nominative: plan
• instrumental: planom

Because it comes after sa in this meaning, the instrumental is required:

• sa planom = with the plan

Both s and sa can mean with, and both can be used with the instrumental case. The choice often depends on pronunciation.

Sa is commonly used before words where s would be awkward to pronounce, especially before certain consonants or consonant clusters.

In modern Serbian, sa planom is the normal form here.

You will often see:

• s bratom / sa bratom depending on style and pronunciation habits
• sa mnom
• sa tobom
• sa planom

For a learner, the safest choice here is simply to use sa planom.

No, not naturally in this context.

Slagati se sa nečim means to agree with something. So Ne slažemo se sa planom means we do not agree with the plan.

When used with people, slagati se s nekim can sometimes mean to get along with someone:

• Dobro se slažemo. = We get along well.

But with planom, the meaning is clearly agree with the plan, not get along with the plan.

Because Serbian usually does not need an explicit subject pronoun when the verb ending already shows who the subject is.

The ending -mo in slažemo tells you the subject is we.

So:

• (Mi) slažemo se. = We agree.
• (Mi) ne slažemo se sa planom. = We do not agree with the plan.

You can add mi for emphasis or contrast, but it is not necessary.

The basic neutral order is:

• Ne slažemo se sa planom.

But Serbian word order is somewhat flexible. You may also hear:

• Sa planom se ne slažemo.
• Ne slažemo se mi sa planom. — with emphasis on we
• Mi se ne slažemo sa planom.

However, se tends to appear near the beginning of the clause, and learners should first get comfortable with the neutral version:

• Ne slažemo se sa planom.

In a simple sentence like this, se usually comes right after the verb form:

• Ne slažemo se.

So the pattern is:

• ne + verb + se

Examples:

• Ne bojim se. = I am not afraid.
• Ne sećamo se. = We do not remember.
• Ne slažemo se. = We do not agree.

Word order with se can be tricky in longer sentences, but in this sentence the placement is standard and natural.

The dictionary form is slagati se.

That is the infinitive, meaning to agree.

Be careful, because Serbian dictionaries may list verbs with se when that particle is part of the normal verb form. Here, you should memorize it as:

• slagati se = to agree

not just slagati.

You just remove ne:

• Slažemo se sa planom.

So:

• Slažemo se sa planom. = We agree with the plan.
• Ne slažemo se sa planom. = We do not agree with the plan.

No. In this sentence, sa planom means with the plan, because it depends on the verb slagati se.

If you want to say according to the plan, Serbian would usually use a different structure, such as:

• po planu = according to the plan

So:

• Ne slažemo se sa planom. = We do not agree with the plan.
• Po planu... = According to the plan...

Those are different meanings and different grammar.