Липкое тесто трудно вымешивать.

Because тесто is neuter singular, and the adjective has to agree with it.

• тесто = neuter singular noun
• липкий = dictionary form of sticky
• neuter singular nominative/accusative form = липкое

So:

• липкий хлеб = sticky bread (masculine)
• липкая масса = sticky mass (feminine)
• липкое тесто = sticky dough (neuter)

Agreement in gender, number, and case is one of the most important things to notice in Russian.

In Russian, the verb to be is normally not expressed in the present tense.

So:

• Липкое тесто трудно вымешивать. literally feels like: Sticky dough difficult to knead.

But the natural English translation uses is:

• Sticky dough is difficult to knead.

This is completely normal in Russian. In past or future, forms of быть would appear if needed, but in the present tense they are usually omitted.

Because трудно here is not an adjective modifying тесто. It is a predicative adverb/state word, meaning something like it is difficult.

Compare:

• трудное тесто = difficult dough
  This sounds odd in most contexts, because it describes the dough itself with an adjective.
• тесто трудно вымешивать = the dough is difficult to knead

So in this sentence:

• липкое describes тесто
• трудно describes the action of kneading it

A good way to feel the structure is:

• [Липкое тесто] [трудно] [вымешивать].
• Sticky dough / is difficult / to knead

Functionally, it is the object of вымешивать, so it is in the accusative case.

However, тесто is a neuter inanimate noun, and for such nouns the accusative looks exactly like the nominative. So the form stays тесто.

That is why this can be confusing. You do not see a special ending, but grammatically it is still the object of the infinitive.

Compare with a feminine noun where the accusative is visible:

• Липкую массу трудно перемешивать.
• Here масса becomes массу in the accusative.

So in your sentence, тесто looks unchanged only because of its noun type.

This is an impersonal kind of construction. Russian often leaves the doer unstated when the point is a general fact.

So:

• Липкое тесто трудно вымешивать. means something like:
• It is hard to knead sticky dough
• Sticky dough is hard to knead

The sentence is not about a specific person. It means for people in general, or in general as an activity, this is difficult.

English does something similar with it is hard to... or with passive-like wording such as X is hard to do.

Because the sentence talks about the action in general, not about a single completed result.

• вымешивать = imperfective
• вымесить = perfective

Here the meaning is:

• sticky dough is difficult to knead / to work thoroughly, as a general process

If you used the perfective вымесить, it would focus more on successfully kneading it through to completion in a specific instance.

Very often after words like трудно, легко, приятно, полезно, Russian uses the imperfective infinitive for general statements:

• Это трудно понять. = This is hard to understand.
• Овощи полезно есть. = Vegetables are good to eat.

Both relate to kneading, but вымешивать usually suggests kneading thoroughly, working something until it reaches the right consistency.

• месить = to knead
• вымешивать = to knead thoroughly / work through by kneading

The prefix вы- often adds a sense of bringing something to a finished or proper result.

So:

• трудно месить = hard to knead
• трудно вымешивать = hard to knead properly / hard to work through thoroughly

In a cooking context, вымешивать is very natural because dough often needs to be kneaded well, not just touched or mixed a little.

Yes. Russian word order is flexible, and changing it usually changes the emphasis, not the core meaning.

Your sentence:

• Липкое тесто трудно вымешивать.

This puts липкое тесто first, so the sentence starts with the thing being discussed: sticky dough.

Other possible orders:

• Трудно вымешивать липкое тесто.
  More like: It is hard to knead sticky dough.
  This feels slightly more neutral as a statement about the action.

• Вымешивать липкое тесто трудно.
  This can emphasize the whole activity: Kneading sticky dough is difficult.

So the original order is perfectly natural, but it is not the only possible one.

Not in the usual grammatical sense.

Semantically, English often treats sticky dough as the thing being described:

• Sticky dough is difficult to knead.

But in Russian, the structure is closer to:

• To knead sticky dough is difficult or
• It is difficult to knead sticky dough

That means:

• вымешивать is the infinitive expressing the action
• трудно is the predicative center
• липкое тесто is the object of the infinitive

So although липкое тесто comes first, it is not really the grammatical subject in the same way an English learner might expect.

Because Russian has no articles.

So тесто can mean:

• dough
• the dough
• some dough
• a dough (depending on context, though this is less common in English)

The exact meaning comes from context, not from a word like the or a.

In this sentence, the sense is general:

• sticky dough in general is hard to knead

Yes, depending on context, that is a very reasonable interpretation.

Russian often leaves some things more open than English does. Without context, the sentence most naturally reads as a general statement:

• Sticky dough is hard to knead.

But in a real cooking situation, it could also refer to a specific batch:

• The sticky dough is hard to knead.

And because of вымешивать, there is also a nuance of kneading thoroughly/properly.

So context determines whether the meaning is:

• general truth
• a comment about a specific dough
• emphasis on kneading thoroughly

All of those fit the grammar.