Es mejor que te hagas una coleta cuando hace calor.

Because es mejor que is an expression of advice, opinion, or recommendation, and that normally triggers the subjunctive in Spanish.

So:

• Es mejor que te hagas... = correct
• Es mejor que te haces... = incorrect

A good rule to remember is:

• Es mejor que + subjunctive
• Es mejor + infinitive is also possible in some cases, but not que + indicative

Hagas is the present subjunctive form of hacer for tú.

The present subjunctive of hacer is:

• yo haga
• tú hagas
• él/ella/usted haga
• nosotros hagamos
• vosotros hagáis
• ellos/ellas/ustedes hagan

It is irregular because it comes from the yo form of the present indicative:

• yo hago → subjunctive stem haga-

Because the expression is hacerse una coleta, which is a reflexive construction.

Here, te refers to the person doing the action to themselves:

• te haces una coleta = you put your hair in a ponytail / you make yourself a ponytail

Spanish often uses reflexive verbs for personal grooming and appearance-related actions, for example:

• lavarse el pelo
• peinarse
• maquillarse

Without te, the sentence would lose that idea of doing it to yourself.

Because with a conjugated verb, object and reflexive pronouns usually go before the verb in Spanish.

So:

• te hagas = correct

Compare that with infinitives, where the pronoun can attach to the end:

• hacerte una coleta

So both patterns are normal, depending on the verb form:

• que te hagas una coleta
• es mejor hacerte una coleta

Because Spanish often omits subject pronouns when they are already clear from the verb form.

In te hagas, both the reflexive pronoun te and the verb ending -as already show that the sentence is talking to you singular.

So:

• Es mejor que te hagas una coleta... = perfectly natural
• Es mejor que tú te hagas una coleta... = possible, but more emphatic

Spanish usually only adds tú if the speaker wants contrast or emphasis.

In Spain, coleta commonly means ponytail.

So hacerse una coleta is a very normal way to say to put your hair in a ponytail.

A few useful notes:

• una coleta = one ponytail
• dos coletas = pigtails / two ponytails

Another expression you may also hear is cola de caballo, but coleta is very common in Spain.

Because una coleta refers to the hairstyle in a general sense: a ponytail.

It is not talking about a specific ponytail already mentioned before, so the indefinite article is natural here.

Compare:

• Hazte una coleta = Put your hair in a ponytail
• Hazte la coleta = Do the ponytail / do your ponytail

The second one sounds more like there is a specific ponytail already understood in the situation. In this sentence, una coleta is the more general, neutral choice.

Because cuando hace calor refers to a general or habitual situation: whenever it is hot.

Here, the speaker is giving general advice, so the indicative is natural:

• Es mejor que te hagas una coleta cuando hace calor.

If you say cuando haga calor, that usually points to a future or not-yet-realised situation:

• Cuando haga calor, hazte una coleta.
  = When it gets hot / when it is hot in the future, put your hair in a ponytail.

So the difference is:

• cuando hace calor = in general, whenever it’s hot
• cuando haga calor = on a future occasion, when it becomes hot

Because Spanish normally uses hacer with weather and temperature expressions:

• hace calor = it’s hot
• hace frío = it’s cold
• hace buen tiempo = the weather is nice

By contrast, estar caliente usually means that a person or thing is physically warm/hot, not that the weather is hot.

So:

• Hace calor = the weather is hot
• La sopa está caliente = the soup is hot

That is why hace calor is the correct choice here.

Yes. That version is also correct, but the nuance is slightly different.

• Es mejor que te hagas una coleta... = direct advice to you
• Es mejor hacerse una coleta... = more general, more impersonal, like a general statement

So both work:

• Es mejor que te hagas una coleta cuando hace calor.
• Es mejor hacerse una coleta cuando hace calor.

The original sentence sounds more like someone speaking directly to another person.

Yes. A very natural alternative would be to use recogerse el pelo, which means to tie one’s hair up / wear one’s hair up.

For example:

• Es mejor que te recojas el pelo cuando hace calor.
• Es mejor recogerse el pelo cuando hace calor.

This is slightly more general than hacerse una coleta, because recogerse el pelo does not specify exactly which hairstyle you use. Hacerse una coleta specifically means making a ponytail.