Sunt împotriva planului.

Sunt is the present-tense form of a fi (to be).

In this sentence, it means I am:

• (Eu) sunt = I am

Romanian often leaves out the subject pronoun when it is clear from the verb form, so sunt by itself usually implies I am here.

Yes, you can say Eu sunt împotriva planului, but Romanian usually drops subject pronouns unless you want emphasis or contrast.

So:

• Sunt împotriva planului = neutral, natural
• Eu sunt împotriva planului = I am against the plan, perhaps contrasting with someone else

This is normal in Romanian and is one of the first things English speakers notice.

Împotriva is a preposition meaning against.

It links the speaker to what they oppose:

• împotriva planului = against the plan

A very important point is that împotriva requires the following noun to be in the genitive form.

Because împotriva takes the genitive, and the genitive form of planul (the plan) is planului.

Here is the pattern:

• plan = plan
• planul = the plan
• planului = of the plan / the plan after a genitive-taking preposition like împotriva

So:

• împotriva planului = against the plan

You cannot normally say împotriva planul.

Yes, planului is definite: it means the plan.

If you want against a plan, you would say:

• împotriva unui plan

Compare:

• Sunt împotriva planului = I am against the plan
• Sunt împotriva unui plan = I am against a plan

So the sentence you were given refers to a specific plan.

Romanian usually puts the definite article at the end of the noun, unlike English.

For example:

• plan = plan
• planul = the plan

Then, when the noun changes case, the ending changes too:

• planului = the genitive/dative form of planul

This attached article is a basic feature of Romanian grammar.

Yes. Contra also means against, and Sunt contra planului is understandable and common.

However:

• împotriva often sounds a bit more standard or neutral
• contra can sound slightly more direct, concise, or stylistically different depending on context

Both are correct in many situations.

Yes, in standard Romanian, împotriva is followed by the genitive.

Examples:

• împotriva planului = against the plan
• împotriva deciziei = against the decision
• împotriva guvernului = against the government

So if you learn împotriva, it is a good idea to learn it together with this rule: împotriva + genitive.

You use the pronoun forms that go with this preposition:

• împotriva mea = against me
• împotriva ta = against you
• împotriva lui = against him
• împotriva ei = against her
• împotriva noastră = against us
• împotriva voastră = against you all
• împotriva lor = against them

So the pattern is not limited to nouns like planului.

Sunt împotriva planului is the normal, neutral order.

Romanian word order is somewhat flexible, but changes usually add emphasis or sound more literary or marked. For example:

• Împotriva planului sunt can be understood, but it is not the most natural everyday order in this context.

For a learner, the safest version is:

• Sunt împotriva planului

A helpful approximation is:

• îm-po-TRI-va

Notes:

• The stress falls on tri
• The î is a Romanian vowel that does not exist in English; it is a central sound, somewhat like a tight, neutral vowel
• mp is pronounced clearly together

If you cannot produce î perfectly yet, that is normal. It takes practice.

A useful approximation is:

• PLA-nu-lui

Notes:

• The stress is usually on pla
• The ending -lui sounds roughly like looy, but not exactly English loo-ee
• Try to keep it as one smooth word: pla-nu-lui

Because Romanian attaches articles and case endings to nouns, longer forms like this are very common.

Yes, sunt can also mean they are:

• eu sunt = I am
• ei/ele sunt = they are

In isolation, context normally tells you which one is meant. In your sentence, the intended meaning is I am against the plan.

If you want to remove all doubt, you can add the pronoun:

• Eu sunt împotriva planului = I am against the plan
• Ei sunt împotriva planului = They are against the plan

It is generally neutral and works well in both spoken and written Romanian.

It is a very natural way to express opposition to something. It does not sound especially formal or especially casual.