Cuando se me acaba el gel, reutilizo el envase pequeño para viajar.

Because this sentence describes a habitual action: something the speaker does whenever the gel runs out.

So:

• Cuando se me acaba el gel, reutilizo... = When/Whenever my gel runs out, I reuse...

In Spanish, for repeated or general situations, it is very normal to use the present tense in both parts.

Compare:

• Cuando se me acaba el gel, reutilizo el envase...
  = a usual routine
• Cuando se me acabe el gel, reutilizaré el envase...
  = one future occasion: When my gel runs out, I’ll reuse the container

So the sentence you have is about a regular habit, not a single future event.

Se me acaba is a very common Spanish way to say that something runs out or gets used up.

Breakdown:

• acabarse = to run out / to be finished / to get used up
• me = to me / on me
• se = part of the verb acabarse here

So se me acaba el gel is roughly:

• my gel runs out
• or more literally, the gel runs out on me

That me often shows the person affected by the situation. Spanish likes this structure a lot with things like food, money, batteries, shampoo, etc.

Examples:

• Se me acabó el café = I ran out of coffee
• Se me acaban las pilas = My batteries are running out

Yes, in this sentence they work together naturally.

• se belongs to the verb acabarse
• me shows who is affected

So:

• Se acaba el gel = The gel runs out / gets used up
• Se me acaba el gel = The gel runs out on me / for me

The me adds the idea that this affects the speaker personally. In everyday Spanish, this is extremely common and sounds very natural.

It does not mean the speaker is doing the action to themselves. It is not really reflexive in the English sense.

Good question. The version with se is the natural one when talking about something being used up.

• acabar often means to finish or to end
• acabarse often means to run out, to come to an end, or to be used up

So:

• Acabo el trabajo = I finish the work
• El trabajo se acaba = The work ends
• Se me acaba el gel = I run out of gel / the gel gets used up on me

In this sentence, acabarse is the right choice because the gel is being consumed until none is left.

Spanish often uses the definite article (el, la, los, las) where English would use a possessive like my.

Here, ownership is already clear from context, especially because of me:

• se me acaba el gel already strongly suggests the gel I use / my gel

So el gel sounds natural and idiomatic.

You could say mi gel, but it would usually sound more marked, for example if you were contrasting it with someone else’s:

• Cuando se me acaba mi gel, uso el tuyo
  = When my gel runs out, I use yours

Without that kind of contrast, el gel is more natural.

Because Spanish normally drops subject pronouns when the verb ending already makes the subject clear.

• reutilizo already means I reuse
• so yo reutilizo is usually unnecessary

Spanish includes yo only when it wants emphasis, contrast, or clarity:

• Yo reutilizo el envase, pero mi hermano lo tira
  = I reuse the container, but my brother throws it away

In your sentence, no emphasis is needed, so just reutilizo is the most natural form.

Envase means container or packaging/container the product comes in.

It is a fairly general word. In this sentence, it refers to the small container that held the gel.

Depending on context, English might translate it as:

• container
• bottle
• pack
• tube

Spanish uses envase when it wants to stay general, rather than naming the exact shape.

For example:

• botella = bottle
• frasco = jar/small bottle
• bote = jar/tub/container
• envase = general container/package

So envase is broad and practical here.

Because in Spanish, descriptive adjectives usually come after the noun.

So:

• envase pequeño = small container

That is the normal, neutral order.

If you put the adjective before the noun, it can sound more literary, emotional, or slightly different in nuance:

• el pequeño envase

This is possible, but it often feels more like the little container or gives it a bit more emphasis.

In everyday neutral Spanish, el envase pequeño is the safer, more standard choice.

It expresses purpose.

• para
  • infinitive = for doing something / in order to do something

So:

• para viajar = for travelling / to travel

In natural English, the whole phrase means something like:

• I reuse the small container for travelling
• I reuse the small container when I travel
• I reuse the small container to take with me on trips

It implies the speaker reuses the small container as a practical travel item.

Yes. That would also be natural.

Both are common:

• se me acaba el gel
• se me termina el gel

Both can mean my gel runs out.

A small nuance:

• acabarse often strongly suggests being used up
• terminarse can also mean to be finished / to come to an end

In many everyday situations, the difference is tiny, and native speakers may choose either one.

For a future one-time event, you would normally use the subjunctive after cuando:

• Cuando se me termine el gel, reutilizaré el envase pequeño.

Not always exactly.

In Spain, gel often refers to shower gel / body wash, though context can also make it mean other kinds of gel, such as hair gel.

So if a learner sees el gel by itself, they may need context to know which type is meant.

In this sentence, because it mentions reusing a small container for travelling, it sounds like a toiletry product, very possibly shower gel or something similar.

The important grammar point is that gel is masculine in Spanish:

• el gel
• un gel pequeño

Yes, reutilizo is correct and natural, but it is a slightly more formal or precise word than some alternatives.

It means:

• I reuse

Possible alternatives:

• vuelvo a usar el envase = I use the container again
• reuso el envase = I reuse the container

In Spain, reutilizar is very common and sounds completely normal, especially in contexts about reducing waste, recycling, or practical habits.

So the original sentence sounds natural and clear.