Если конверт слишком маленький, документы лучше нести в папке, а не в руках.

Если means if and introduces a condition.

In this sentence, it works just like a normal real-world condition:

Если конверт слишком маленький, документы лучше нести в папке...
= If the envelope is too small, it’s better to carry the documents in a folder...

This is not a special “unreal” or “contrary-to-fact” construction. Russian often uses если very straightforwardly for practical situations like this.

Маленький agrees with конверт.

• конверт = masculine singular
• so the adjective must also be masculine singular:
  • маленький

Here, конверт is the subject of the clause, so it is in the nominative case:

• конверт
• маленький

That is why you get:

конверт слишком маленький
= the envelope is too small

Слишком means too, in the sense of more than is acceptable / more than is useful.

So:

• очень маленький = very small
• слишком маленький = too small

In this sentence, слишком is important because it explains why a different action is recommended. The envelope is not just small; it is too small for the documents.

Here документы is the direct object of нести.

The verb нести takes the accusative case. However, документы is an inanimate plural noun, and for inanimate plural nouns, the accusative form is the same as the nominative form.

So:

• nominative plural: документы
• accusative plural: документы

That is why the form looks unchanged.

Лучше means better and is commonly used with an infinitive to express what is advisable or preferable.

So:

• лучше нести = it is better to carry
• more naturally in English here: it’s better to carry

This is a very common Russian pattern:

• Лучше подождать. = It’s better to wait.
• Лучше поехать утром. = It’s better to go in the morning.

Notice that Russian often does not need a separate word like it is. The idea is understood from context.

This is a very common learner question, because Russian distinguishes between two related verbs:

• нести = to carry, to be carrying, in one direction / in a specific instance
• носить = to carry regularly, habitually, or in various directions

In this sentence, the meaning is about a practical single action: how to carry the documents in this situation. That is why нести is the natural choice.

Compare:

• Документы лучше нести в папке.
  = It’s better to carry the documents in a folder.
  (this particular act of carrying)

• Он всегда носит документы в папке.
  = He always carries documents in a folder.
  (habitual action)

Russian often omits the subject when the meaning is general, impersonal, or obvious from context.

Here, документы лучше нести в папке does not literally say you should carry the documents, but that is the natural meaning in context. It is a general recommendation:

• it’s better to carry the documents in a folder
• you’d better carry the documents in a folder

Russian often expresses advice this way without naming the person directly.

The preposition в here means in.

After в, when you mean location, Russian uses the prepositional case. The noun папка changes like this:

• dictionary form: папка
• prepositional singular: в папке

So:

• в папке = in a folder

This tells you where the documents should be carried.

В руках is the normal expression for in the hands / in one’s hands.

The noun рука is plural here because the idea is usually in one’s hands rather than in one hand.

Forms:

• singular: рука
• plural: руки
• prepositional plural after в: в руках

So:

• в руках = in (one’s) hands

Using на руках would usually mean something different, depending on context, such as on one’s hands/arms or in certain idiomatic expressions. For physically holding something, в руках is the standard phrase.

А не is very common in Russian when contrasting two alternatives:

• в папке, а не в руках
• in a folder, not in one’s hands

Here а marks a contrast or correction: this way, not that way.

Compare the feeling:

• а не = rather than / and not / instead of
• но = but

In this sentence, а не sounds more natural because the speaker is setting one option against another.

Yes, Russian word order is flexible, and other arrangements are possible. But this order is very natural.

The sentence begins with the condition:

Если конверт слишком маленький...
= If the envelope is too small...

Then it gives the recommendation:

документы лучше нести в папке, а не в руках.

This structure is clear and common: first the situation, then the advice.

Russian could rearrange parts for emphasis, but the version given is the most neutral and natural for ordinary speech or writing.

Конверт is the standard word for envelope.

A learner might wonder whether there is some special business or document word here, but no: конверт is the normal everyday noun.

So:

• конверт = envelope

The sentence simply says that if the envelope is too small, a folder is the better way to carry the documents.