Podemos ir a la playa siempre que haga buen tiempo.

Because siempre que is introducing a condition about the future or a possible situation, Spanish normally uses the subjunctive.

So:

• siempre que haga buen tiempo = as long as the weather is good
• haga is the present subjunctive of hacer

A native English speaker often expects the normal present tense here, but Spanish uses the subjunctive after expressions like siempre que when the situation is not presented as a known fact.

Compare:

• Podemos ir a la playa siempre que haga buen tiempo.
  The good weather is only a condition.

• Siempre que hace buen tiempo, vamos a la playa.
  This means Whenever the weather is good, we go to the beach — a habitual fact.

Here, siempre que means as long as, provided that, or so long as.

Even though siempre usually means always, the expression siempre que is a fixed phrase. It does not literally mean always that in normal English.

Depending on context, siempre que can mean:

• as long as / provided that
• whenever

In this sentence, the conditional meaning fits best:
We can go to the beach as long as the weather is good.

Because it often introduces a condition that has not yet been confirmed.

Spanish uses the subjunctive after many conjunctions when the speaker is talking about:

• something uncertain
• something in the future
• something hypothetical
• something dependent on a condition

In siempre que haga buen tiempo, the speaker is not saying the weather is definitely good. They are saying the plan depends on that condition being met.

That is why haga is used.

Haga is the present subjunctive form of hacer for él/ella/usted.

The full present subjunctive of hacer is:

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

In weather expressions, Spanish often uses an understood impersonal it idea, so haga buen tiempo is like saying if it is good weather or more literally if it makes good weather.

Spanish often uses hacer for weather expressions where English uses to be.

Examples:

• Hace frío = It is cold
• Hace calor = It is hot
• Hace sol = It is sunny
• Hace viento = It is windy
• Hace buen tiempo = The weather is good
• Hace mal tiempo = The weather is bad

So in your sentence, haga buen tiempo comes from that weather pattern.

Because bueno becomes buen before a singular masculine noun.

This is called apocopation.

So:

• un buen día
• buen tiempo
• un buen libro

But after the noun, you use bueno:

• El tiempo es bueno
• Ese libro es bueno

Since tiempo is a singular masculine noun and bueno comes before it, it changes to buen.

It can mean both, depending on context.

Very often, learners are confused because tiempo is used for:

• time
• weather

Here it clearly means weather because of the expression hacer buen tiempo.

Examples:

• No tengo tiempo. = I don’t have time.
• Hace buen tiempo. = The weather is good.

So context tells you which meaning is intended.

Because ir a is the normal way to say to go to a place.

So:

• ir a Madrid
• ir al cine
• ir a la playa

The preposition a marks the destination.

A common English-speaker mistake is trying to use something like in or leaving the preposition out, but in Spanish ir normally needs a before the destination.

Because in Spanish, destinations like the beach, the cinema, the park, etc. often take the definite article.

So Spanish naturally says:

• ir a la playa
• ir al parque
• ir al cine

Even if English sometimes says just go to the beach with the, Spanish also uses the article here.
A playa by itself would sound incomplete or unnatural in standard Spanish.

Because after poder, the next verb normally stays in the infinitive.

So:

• puedo comer
• puedes venir
• podemos ir

This is the same basic pattern as English modal verbs:

• can go
• can eat
• can come

So podemos ir literally works like we can go.

It can suggest several related ideas, depending on context:

• ability: we are able to go
• possibility: it’s possible for us to go
• permission/allowance: we may go
• suggestion: we could go

In this sentence, it most naturally sounds like possibility or a suggestion:

• We can go to the beach, as long as the weather is good.

So it is less about physical ability and more about whether the plan is possible.

Yes, you could say:

• Podemos ir a la playa si hace buen tiempo.

That means We can go to the beach if the weather is good.

The difference is mainly nuance:

• si = if
• siempre que = as long as / provided that

So siempre que sounds a bit more like the trip depends on that condition being satisfied.

Both are natural, but they are not exactly identical.

Yes. Siempre y cuando is very similar and also takes the subjunctive.

For example:

• Podemos ir a la playa siempre y cuando haga buen tiempo.

This also means as long as / provided that the weather is good.

Compared with siempre que, siempre y cuando can sound a little more formal or a little more emphatic, but both are common and correct.

By itself, it could be understood either way, depending on context.

It might mean:

• a plan for one occasion: We can go to the beach as long as the weather is good
• a general rule: We can go to the beach whenever the weather is good

What makes it feel more conditional is the use of siempre que + subjunctive, which often points to a condition that must be met.

If the speaker wanted to describe a repeated fact more clearly, they might say:

• Siempre que hace buen tiempo, vamos a la playa.

That is more clearly Whenever the weather is good, we go to the beach.

Yes. Spanish allows some flexibility.

For example:

• Podemos ir a la playa siempre que haga buen tiempo.
• Siempre que haga buen tiempo, podemos ir a la playa.

Both are correct.

The second version puts more emphasis on the condition first. English can do the same:

• As long as the weather is good, we can go to the beach.

So the meaning stays basically the same, but the focus shifts slightly.

Podemos ir and vamos a poder ir are close, but not identical.

• Podemos ir = we can go
• Vamos a poder ir = we’re going to be able to go

The second one sounds a bit more explicitly future-oriented.

So:

• Podemos ir a la playa siempre que haga buen tiempo
  sounds like a present possibility or plan.

• Vamos a poder ir a la playa siempre que haga buen tiempo
  stresses future ability/possibility more clearly.

Both are natural, but podemos ir is simpler and very common.