Если в воскресенье будет тепло, мы будем гулять по тихой аллее.

That is a very common difference between English and Russian.

In real future conditions, Russian normally uses the future in both parts of the sentence:

• Если в воскресенье будет тепло, мы будем гулять...

Literally, this is closer to:

• If on Sunday it will be warm, we will be walking...

English does not do that, but Russian does.

So:

• Если будет тепло = If it is warm / if it turns out to be warm
• Если будет дождь = If it rains / if there is rain
• Если я приду = If I come

This is completely normal Russian grammar.

Тепло here is not an adjective describing a noun. It is a predicative word meaning warm in the sense of the weather/atmosphere is warm.

So:

• тёплый = warm as an adjective, used with a noun
  
  • тёплый день = a warm day
• тепло = it is warm / warm conditions

In this sentence:

• будет тепло = it will be warm

Russian does not need a dummy subject like English it. English says it will be warm, but Russian just says будет тепло.

Yes. В воскресенье means on Sunday.

Russian often uses в + accusative for a specific point in time:

• в понедельник = on Monday
• в среду = on Wednesday
• в воскресенье = on Sunday

Here, воскресенье is a neuter noun, and its accusative form looks the same as the nominative, so the word does not visibly change.

A useful contrast:

• в воскресенье = on Sunday (one specific Sunday)
• по воскресеньям = on Sundays / every Sunday

Будем гулять is the compound future of the imperfective verb гулять.

• гулять = imperfective
• будем гулять = we will be walking / we will be out for a walk

This form focuses on the process or activity itself, not on a completed result.

So here the idea is something like:

• we’ll be walking
• we’ll spend time walking
• we’ll go for a walk and be walking along the alley

By contrast, погуляем is a perfective future and often sounds more like:

• we’ll go for a walk
• we’ll take a walk for a while

Both can be possible in context, but будем гулять emphasizes the ongoing activity more clearly.

This is mostly an aspect question.

Будем гулять

• imperfective future
• emphasizes the activity/process
• can feel a bit more descriptive or neutral

Погуляем

• perfective future
• presents the action more as a single whole event
• often suggests go for a walk for some time

So:

• Если будет тепло, мы будем гулять по тихой аллее
  = If it’s warm, we’ll be walking along the quiet alley.

• Если будет тепло, мы погуляем по тихой аллее
  = If it’s warm, we’ll take a walk along the quiet alley.

Both are natural, but they spotlight the action differently.

По тихой аллее uses the dative case.

Here:

• по = along / on / around, depending on context
• тихой = dative singular feminine of тихая
• аллее = dative singular of аллея

So:

• по тихой аллее = along the quiet alley

This is a very common pattern:

• идти по улице = to walk along the street
• гулять по парку = to walk around the park
• бежать по дороге = to run along the road

A learner may expect the accusative here because of movement, but with по in this meaning, Russian normally uses the dative.

In this sentence, по means something like:

• along
• down
• through (in some contexts)

So гулять по тихой аллее means walking along the quiet alley.

This is one of the common uses of по with the dative:

• movement over, along, or around an area

Examples:

• ходить по комнате = walk around the room
• идти по мосту = go across/along the bridge
• гулять по набережной = walk along the embankment

So here по does not mean according to or after, which are other possible meanings of по in different contexts.

Because the sentence has a subordinate clause introduced by если.

Russian separates this kind of clause with a comma:

• Если в воскресенье будет тепло, мы будем гулять по тихой аллее.

This is the same idea as:

• When he comes, we will start.
• If it rains, we will stay home.

In Russian, the comma is required whether the если clause comes first or second:

• Если в воскресенье будет тепло, мы будем гулять по тихой аллее.
• Мы будем гулять по тихой аллее, если в воскресенье будет тепло.

Yes. Russian word order is more flexible than English word order.

A very natural alternative is:

• Мы будем гулять по тихой аллее, если в воскресенье будет тепло.

The basic meaning stays the same.

The version with Если ... first usually sets up the condition first:

• If it’s warm on Sunday, ...

The version with the main clause first can sound a little more like the speaker starts with the plan and then adds the condition:

• We’ll walk along the quiet alley, if it’s warm on Sunday.

So the choice is often about focus and flow, not a big difference in meaning.

Yes. Russian often omits personal pronouns when the subject is clear from the verb ending.

So you could say:

• Если в воскресенье будет тепло, будем гулять по тихой аллее.

That still means If it’s warm on Sunday, we’ll walk along the quiet alley, assuming the subject is understood from context.

Using мы is also perfectly normal. It can make the subject more explicit, clearer, or slightly more emphatic.

So:

• мы будем гулять = explicit we
• будем гулять = we will walk, with we understood

Because если expresses a condition: it may happen, or it may not.

• Если в воскресенье будет тепло... = If it is warm on Sunday...

That means the warmth is not guaranteed.

Когда means when, and usually suggests that the speaker treats the event as expected or certain in some way.

So:

• Если будет тепло = if it’s warm
• Когда будет тепло = when it’s warm / when it becomes warm

In this sentence, the speaker is making a plan that depends on the weather, so если is the natural choice.