Липкое тесто осело, оттого что его мало складывали.

Because they agree with тесто, which is a neuter singular noun.

• тесто = neuter singular
• липкое = neuter singular form of липкий
• осело = past tense, neuter singular form of осесть

In Russian, adjectives agree with the noun they describe, and in the past tense, verbs also agree with the subject in gender and number.

So:

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

And similarly:

• хлеб осел
• масса осела
• тесто осело

Осело is the past tense of осесть. In this context, it means something like:

• collapsed
• sank
• fell flat
• subsided

When talking about dough, it usually means that the dough did not hold its structure and went flat / sank down.

This verb is perfective, so it presents the result: the dough ended up in a collapsed state.

Оттого что is a conjunction meaning because, since, or from the fact that.

It is very close in meaning to потому что. In many sentences, either one would work.

So this sentence could also be written as:

Липкое тесто осело, потому что его мало складывали.

A rough difference:

• потому что = the most common neutral because
• оттого что = a bit more explanatory or literary, sometimes with a slight sense of as a result of the fact that

In this sentence, both are natural.

Because оттого что introduces a subordinate clause giving the reason.

Main clause:

• Липкое тесто осело

Reason clause:

• оттого что его мало складывали

Russian normally puts a comma before conjunctions like что, потому что, оттого что, when they introduce a subordinate clause.

Because его is the accusative form of the pronoun оно.

Here, его means it and is the direct object of складывали.

Compare:

• Оно осело. = It collapsed.
  Here it is the subject.
• Его складывали. = They folded it.
  Here it is the object.

This can feel strange to English speakers because тесто itself does not change in the accusative:

• Я вижу тесто.

But personal pronouns do change:

• оно → его in the accusative

So его here is completely normal.

This is a very common Russian pattern called an indefinite-personal construction.

Literally, складывали is they were folding / they folded, but Russian often uses 3rd person plural with no stated subject to mean:

• someone folded it
• people folded it
• it was folded

So:

его мало складывали
literally: they folded it little
natural English: it was not folded enough

Russian often prefers this kind of active plural wording where English might prefer a passive.

In baking, складывать тесто means to fold the dough.

That is a technical baking action: you stretch or lift part of the dough and fold it over itself to build structure.

So in this sentence, складывали does not mean stacking objects. It means folding the dough during preparation.

This is a good example of how a verb’s meaning depends heavily on context.

Because складывали is imperfective, and that fits the idea of insufficient repeated/ongoing action.

The sentence is talking about the dough failing because, over the course of preparation, it was not folded enough. That naturally suggests process or repetition, so the imperfective is the normal choice.

• складывали = were folding / used to fold / folded (as a process)
• сложили = folded once to completion, or completed the action

With мало, Russian often prefers the imperfective when talking about an action done too little, not enough, or insufficiently.

Here мало means too little or not enough.

It is modifying the verb складывали, not the noun тесто. So the idea is:

• they folded it too little
• it was folded not enough

This is why a natural English translation is not usually they folded it little, but rather:

• they didn’t fold it enough
• it was under-folded

So мало here expresses insufficient degree or amount of action.

In this sentence, оттого что should be understood as a fixed conjunction meaning because.

You do not need to analyze it word-by-word while reading.

However, there is a useful distinction:

• оттого что = fixed conjunction, because
• от того, что = literally from that / because of that which..., where the words are functioning separately

In your sentence, it is the conjunction: оттого что.

Yes. Russian word order is flexible, though not random.

This sentence could be rearranged, for example:

• Тесто осело, оттого что его мало складывали.
• Оттого что его мало складывали, липкое тесто осело.

The original version emphasizes липкое тесто right away.

Russian word order often reflects:

• what is already known
• what is being emphasized
• what sounds natural in context

So the meaning stays basically the same, but the focus can shift a little.

Because Russian has no articles like a or the.

So липкое тесто can mean:

• sticky dough
• the sticky dough

The exact meaning depends on context.

English speakers often look for a direct equivalent of the, but Russian simply does not mark that distinction with an article. Context does the job instead.