Тесто нужно долго вымешивать.

Yes, тесто is functioning as the direct object of вымешивать, so it is in the accusative.

The reason it looks like the nominative is that тесто is a neuter inanimate noun, and for many such nouns, the nominative and accusative forms are identical.

• nominative: тесто
• accusative: тесто

So in this sentence, the form is accusative by function, even though it looks the same as nominative.

Нужно is an impersonal modal word meaning something like it is necessary, one must, or it needs to be done.

In Russian, this kind of sentence often has no explicit subject. It expresses a general necessity:

• Тесто нужно долго вымешивать. = It is necessary to knead the dough for a long time.

This is a very common Russian structure:

• нужно + infinitive
• надо + infinitive

For example:

• Нужно подождать. = It is necessary to wait.
• Надо работать. = One must work.

Because after нужно, Russian normally uses the infinitive to say what needs to be done.

So the structure is:

• нужно + infinitive

Here:

• нужно вымешивать = it is necessary to knead

This is similar to English patterns like:

• need to do
• have to do

Examples:

• Нужно читать. = One needs to read.
• Нужно приготовить ужин. = Dinner needs to be prepared.

Вымешивать is imperfective, while вымесить is perfective.

The imperfective is used here because the sentence focuses on the process and especially on the duration:

• долго = for a long time

Russian usually prefers the imperfective with words like долго, because it emphasizes an ongoing action rather than a completed result.

So:

• Тесто нужно долго вымешивать. = The dough needs to be kneaded for a long time.

If you used вымесить, the focus would shift more toward completing the action successfully.

The prefix вы- often gives the idea of doing something thoroughly, completely, or to the proper result.

With dough, вымешивать / вымесить means not just mix, but knead thoroughly until it reaches the right consistency.

Compare:

• мешать = to stir, mix
• вымешивать = to knead thoroughly / work through by kneading

So this verb is more specific than just мешать.

Долго is an adverb meaning for a long time.

It modifies the verb:

• долго вымешивать = to knead for a long time

It does not mean after a long time.
For that idea, Russian would use something else, depending on context.

Here it is purely about duration of the action.

It is general.

Russian often uses impersonal sentences like this for:

• instructions
• recipes
• advice
• general rules

So the meaning is something like:

• the dough needs to be kneaded for a long time
• you should knead the dough for a long time
• one must knead the dough for a long time

The sentence does not name who should do it.

Yes, Russian word order is fairly flexible.

These are all possible, with slightly different emphasis:

• Тесто нужно долго вымешивать.
• Нужно долго вымешивать тесто.
• Долго нужно вымешивать тесто.

The version you were given puts тесто first, which can make it sound like the topic is the dough.

A more neutral instructional order is often:

• Нужно долго вымешивать тесто.

Because тесто has the same nominative and accusative form, putting it after the verb can also make the sentence structure feel clearer to learners.

Yes. In many contexts, надо and нужно are very close in meaning.

So you can say:

• Тесто надо долго вымешивать.

This sounds natural too.

Very broadly:

• надо is often a bit more conversational
• нужно can sound a bit more neutral or formal

But in everyday usage, they are often interchangeable.

Yes, you could say:

• Тесто нужно долго месить.

That is also natural and common.

The difference is mainly in nuance:

• месить = to knead
• вымешивать = to knead thoroughly, to work the dough properly

So вымешивать sounds a bit more specific and emphasizes doing the kneading well and fully.

Russian often prefers an active infinitive construction with нужно rather than a passive form.

So instead of saying something like the dough needs to be kneaded in a passive way, Russian commonly says:

• Тесто нужно долго вымешивать.

This is very natural Russian.

A more passive-sounding version is possible in some contexts, but it is usually less straightforward and often less idiomatic than the simple нужно + infinitive pattern.

It can work as any of those, depending on context, but most commonly it sounds like:

• a cooking instruction
• a piece of advice
• a general rule

For example, in a recipe or baking explanation, it means that proper dough preparation requires long kneading.

So grammatically it expresses necessity, but in real use it often functions as practical instruction.