Вот бы дождь начался только ночью, а днём было бы сухо и тепло.

Вот бы is a wish-expression. In this sentence it means something like:

• if only
• I wish
• it would be nice if

So Вот бы дождь начался только ночью... expresses a hoped-for situation, not a factual statement.

Also, вот here does not literally mean here. In this pattern, вот бы works together as a fixed way to express a wish.

Not quite.

A sentence with если бы usually sets up a condition, often followed by a result:

• Если бы пошёл дождь, мы бы остались дома.
  If it rained, we would stay home.

But Вот бы... is more directly a wish:

• Вот бы пошёл дождь.
  I wish it would rain.

So your sentence is not really if X, then Y. It is more like one continuous wish about ideal weather.

Because Russian forms the conditional/subjunctive with:

• a past tense form
• plus the particle бы

So начался by itself means started/began, but in this sentence the idea is not past time. The wish meaning comes from бы.

Here, бы appears earlier in вот бы, but it still affects the clause:

• Вот бы дождь начался...

This is understood as a wish: if only the rain would start...

So the past form here is grammatical, not truly past in meaning.

Because бы is quite mobile in Russian.

It often comes after the first important word or phrase in a clause, and it does not always have to stand directly next to the verb. In this sentence, it is part of the fixed expression вот бы:

• Вот бы дождь начался...

That is completely natural.

You could also see бы in other positions in other sentences, for example:

• Дождь бы начался ночью.
• Начался бы дождь ночью.

But with a wish, вот бы is especially common and idiomatic.

Because начнётся is a normal future-tense fact statement:

• Дождь начнётся ночью.
  The rain will start at night.

But the speaker is not predicting. The speaker is wishing. For that, Russian uses the conditional/subjunctive pattern:

• дождь начался бы
• or here, with the particle earlier: вот бы дождь начался

So начнётся would sound too factual, while начался with бы gives the unreal, wished-for meaning.

Good question. The choice changes the nuance.

• дождь начался = the rain started / would start
• дождь шёл = the rain was falling / would be raining

So начался focuses on the beginning of the rain. It suggests: let the rain only begin at night, not earlier.

If you said:

• Вот бы дождь шёл только ночью...

that would mean: I wish it rained only at night — focusing more on the rain happening during the night and not during the day.

Both can work, but they are slightly different in emphasis. Your sentence highlights the onset of the rain at night.

This is the normal Russian way to say it would be dry and warm.

A few things are happening here:

1. было бы is the conditional form of быть
   = would be

2. сухо and тепло are not regular agreeing adjectives here. They are words used in impersonal weather/state expressions.

So Russian says:

• Было сухо. = It was dry.
• Будет тепло. = It will be warm.
• Было бы тепло. = It would be warm.

This is a very common weather pattern.

Because Russian often uses impersonal constructions for weather, temperature, and general conditions.

English uses it:

• it is warm
• it is dry

Russian often simply says:

• тепло
• сухо
• было бы тепло
• было бы сухо

There is no need for a word corresponding to English it.

That is why the verb appears in the neuter singular form было: it is the standard form for many impersonal sentences.

Because here the sentence describes a general state or weather condition, not the quality of a specific noun.

Compare:

• день тёплый = the day is warm
  Here тёплый is an adjective agreeing with день.

But:

• днём тепло = it is warm during the day
  Here тепло is a state word / predicative adverb.

Likewise:

• воздух сухой = the air is dry
• на улице сухо = it is dry outside

In your sentence, the second pattern is used.

These are standard time expressions meaning:

• ночью = at night / during the night
• днём = during the day / in the daytime

Historically and grammatically, they come from case forms, and you can think of them as fixed adverbial expressions.

This is very common in Russian for parts of the day:

• утром = in the morning
• днём = in the daytime
• вечером = in the evening
• ночью = at night

So a learner should mostly memorize them as time adverbs.

Только means only, and here it limits the time:

• только ночью = only at night

So the idea is that the rain should happen only at night, not in the daytime.

In other words, только is linked to the time phrase, even though in Russian word order can be somewhat flexible.

Because а often marks a contrast:

• rain at night
• but dry and warm in the daytime

So а fits the contrast between night and day very well.

Using и would sound more like simple addition. Using а highlights the opposition between the two time periods.

Yes, Russian word order is flexible, and moving words changes emphasis more than basic meaning.

For example, these are possible in principle:

• Вот бы дождь начался только ночью...
• Вот бы дождь только ночью начался...
• Вот бы только ночью дождь начался...

They all express roughly the same wish, but the emphasis shifts:

• только ночью later in the clause sounds neutral
• moving только ночью earlier gives that idea extra focus

The original version sounds natural and balanced.