Nous doutons que la baignoire soit assez grande, alors nous irons la revoir demain.

Because douter que normally triggers the subjunctive in French.

• être in the present subjunctive for il/elle/on is soit
• So la baignoire soit assez grande means that the bathtub be big enough

French uses the subjunctive after expressions of doubt, uncertainty, emotion, necessity, and similar ideas. Since nous doutons que... expresses doubt, soit is the expected form.

If you said la baignoire est assez grande, that would sound like a straightforward statement of fact, not something uncertain.

Soit is the third-person singular present subjunctive of être.

Here is the present subjunctive of être:

• que je sois
• que tu sois
• qu’il / elle / on soit
• que nous soyons
• que vous soyez
• qu’ils / elles soient

Since la baignoire is singular and feminine, French uses the same verb form as elle, so soit is correct.

Because grande agrees with la baignoire, which is a feminine singular noun.

• masculine singular: grand
• feminine singular: grande

Since baignoire is feminine, the adjective must also be feminine:

• la baignoire est grande
• la baignoire soit assez grande

This agreement happens whether the verb is in the indicative or the subjunctive.

Assez means enough or quite, depending on context. Here it means enough:

• assez grande = big enough

It comes before the adjective it modifies:

• assez grand
• assez grande
• assez intéressant

So la baignoire soit assez grande means the issue is whether the bathtub is sufficiently large.

That la is a direct object pronoun replacing la baignoire.

Instead of repeating the noun, French uses the pronoun:

• Nous irons revoir la baignoire demain.
• Nous irons la revoir demain.

So la means it, referring back to la baignoire.

Because the pronoun is the object of the infinitive revoir.

In French, object pronouns usually go directly before the verb they belong to. Since the action is revoir the bathtub, the pronoun goes before revoir:

• revoir la baignoire
• la revoir

And because irons is followed by an infinitive, the pronoun stays attached to that infinitive idea:

• nous irons la revoir
• literally, we will go see it again

This is similar to:

• Je veux le voir.
• Nous allons les acheter.

Literally, revoir means to see again.

In this sentence, it likely means something like:

• to go look at it again
• to inspect it again
• to check it again

So it does not necessarily mean a casual see again. In context, it sounds more like revisiting the bathtub to judge its size more carefully.

Nous irons is the simple future of aller and means we will go.

French has more than one way to talk about the future:

• nous irons = simple future
• nous allons aller or more naturally nous allons la revoir = near future

Here, nous irons la revoir demain is a normal, natural way to say that tomorrow they will go back to see it again. It sounds a bit more straightforward and written than the near future, but both can work depending on context.

Here alors means so, therefore, or in that case.

It connects the two ideas:

• Nous doutons que la baignoire soit assez grande
• alors nous irons la revoir demain

So the logic is: We have doubts about its size, so we’ll go see it again tomorrow.

Depending on tone and context, alors can also mean then, but here so is the best fit.

Yes, absolutely.

You could say: Nous doutons que la baignoire soit assez grande, alors nous irons revoir la baignoire demain.

That is grammatically correct, but it sounds repetitive. French, like English, usually prefers a pronoun once the thing has already been mentioned.

So nous irons la revoir demain is more natural.

In standard French, yes, usually.

Examples:

• Je doute qu’il vienne.
• Nous doutons qu’elle soit prête.
• Ils doutent que ce soit possible.

The reason is that douter que expresses uncertainty.

One thing to watch out for is the negative:

• Je ne doute pas qu’il viendra is common
• Je ne doute pas qu’il vienne also exists, especially in more formal or careful French

So with plain douter que, you should strongly expect the subjunctive.

Because French uses douter que, not douter si, for this structure.

English often says We doubt whether... or We doubt that..., but French normally uses:

• douter que + subjunctive

So the natural French pattern is:

• Nous doutons que la baignoire soit assez grande.

Using si here would not be the normal structure.

The sentence has two main parts joined by alors:

1. Nous doutons que la baignoire soit assez grande

   • main clause: Nous doutons
   • subordinate clause: que la baignoire soit assez grande

2. alors nous irons la revoir demain

   • alors links the result or consequence
   • main clause: nous irons
   • infinitive phrase: la revoir
   • time expression: demain

So the overall pattern is:

[statement of doubt] + [result/action to take later]

That is why the sentence feels very logical and natural.