Esta botella tiene medio litro de agua, pero yo prefiero llevar un litro cuando hace calor.

Because botella is a feminine noun in Spanish, so the demonstrative has to match it.

• esta botella = this bottle
• este libro = this book

Common forms:

• este = this, masculine singular
• esta = this, feminine singular
• estos = these, masculine plural
• estas = these, feminine plural

So esta botella is correct because botella is feminine.

In this sentence, medio means half, so medio litro means half a litre.

In Spanish, when talking about a measure like this, you normally say:

• medio litro = half a litre
• medio kilo = half a kilo
• media hora = half an hour

You usually do not say un medio litro here.

Also notice agreement:

• medio litro because litro is masculine
• media botella because botella is feminine

Because Spanish normally uses de between a quantity and the thing being measured.

So:

• un litro de agua = a litre of water
• medio litro de agua = half a litre of water
• dos kilos de manzanas = two kilos of apples

This works very much like English of in a litre of water.

Yes. In Spanish, tener is very natural for talking about what a container holds.

So:

• La botella tiene agua = The bottle has water
• La caja tiene libros = The box has books

English often prefers contains in some situations, but Spanish commonly uses tener in everyday speech.

No, yo is not necessary here. Spanish often leaves out subject pronouns because the verb form already shows who the subject is.

• prefiero already means I prefer

So you could say:

• pero prefiero llevar un litro...

Adding yo gives extra emphasis or contrast:

• ..., pero yo prefiero... = ..., but I prefer...

Here it helps create a contrast with the first idea about the bottle.

Because preferir is a stem-changing verb in the present tense. The e in the stem changes to ie in most forms.

Present tense of preferir:

• yo prefiero
• tú prefieres
• él/ella prefiere
• nosotros preferimos
• vosotros preferís
• ellos prefieren

So the sentence uses prefiero because it is the yo form.

Here, llevar means to carry or to take with you.

So:

• prefiero llevar un litro = I prefer to carry/take a litre with me

It does not mean drinking it at that moment.

Compare:

• llevar = to carry, wear, take along
• beber = to drink
• tomar = to take, have, drink

In this sentence, the speaker is talking about the amount of water they want to bring, so llevar is the right verb.

Because Spanish usually needs an article or number with a singular countable noun.

So:

• un litro = a litre
• medio litro = half a litre

You generally cannot say just llevar litro in this context.

Literally, it means when it makes heat, but in natural English it means when it’s hot.

Spanish often uses hacer in weather expressions:

• hace calor = it’s hot
• hace frío = it’s cold
• hace sol = it’s sunny
• hace viento = it’s windy

So cuando hace calor is a very common way to say when the weather is hot.

Because weather expressions in Spanish often use hacer, not ser.

You say:

• Hace calor
• Hace frío

Not:

• Es calor
• Es frío for weather

This is just one of those common Spanish patterns that needs to be learned as a set expression.

The phrase cuando hace calor works as a time clause: when it’s hot. It comes naturally after the main idea it modifies.

• yo prefiero llevar un litro cuando hace calor = I prefer to carry a litre when it’s hot

You could move it earlier for emphasis:

• Cuando hace calor, yo prefiero llevar un litro

Both are correct, but the version in the sentence sounds very natural and neutral.

Yes, in many contexts llevarme could work, especially if you want to stress taking it with me.

• prefiero llevar un litro = I prefer to carry/take a litre
• prefiero llevarme un litro = I prefer to take a litre with me

The version without me is simpler and very natural here. The version with me adds a slight sense of personal involvement or movement away with it.

Because pero joins two contrasting ideas, and in Spanish it is normal to put a comma before it in a sentence like this.

The two ideas are:

• Esta botella tiene medio litro de agua
• yo prefiero llevar un litro cuando hace calor

The comma helps separate the contrast clearly:

• this bottle has half a litre of water,
• but I prefer to carry a litre when it’s hot.

Yes. Agua is a feminine noun, but in the singular it often uses el instead of la when the stressed first sound is a-.

So you say:

• el agua fría
• el alma pura

But the noun is still feminine, so the adjective stays feminine:

• el agua fría, not el agua frío

In your sentence, this does not matter because agua comes after de, so you simply get:

• medio litro de agua

Yes, but it changes the tone slightly.

• pero = but
• sin embargo = however / nevertheless

Your sentence with sin embargo would sound a bit more formal:

• Esta botella tiene medio litro de agua; sin embargo, yo prefiero llevar un litro cuando hace calor.

For everyday speech, pero is the most natural choice here.