No sabía que un conejo pudiera ser tan tranquilo dentro de casa.

Because sabía is the imperfect, and here it describes an ongoing state of not knowing: I didn’t know / I wasn’t aware.

If you used supe, the meaning would change. Supe often means I found out or I learned at a specific moment.

• No sabía que... = I didn’t know that...
• Supe que... = I found out that...

So No sabía is the natural choice for expressing a previous lack of knowledge.

Because after No sabía que..., Spanish often uses the subjunctive when the information was unknown, uncertain, or not being presented as an established fact from that earlier point of view.

Here, pudiera is the imperfect subjunctive of poder.

The sentence is not just saying a rabbit was calm; it is saying I didn’t know that a rabbit could be that calm. That idea is being viewed as something the speaker did not yet know or accept at the time.

This is because of sequence of tenses.

The main verb is in the past: No sabía.
When the main verb is in a past tense, Spanish normally uses the imperfect subjunctive in the subordinate clause, not the present subjunctive.

So:

• No sé que pueda... would be unusual anyway, but if the main idea were in the present, a present subjunctive might appear in other contexts.
• No sabía que pudiera... fits the past-time frame.

Sometimes yes, but the nuance changes.

• No sabía que un conejo podía ser tan tranquilo... sounds more factual: I didn’t know that a rabbit could be so calm...
• No sabía que un conejo pudiera ser tan tranquilo... sounds a bit more tied to the speaker’s earlier uncertainty or surprise.

In real Spanish, both patterns can appear, but the subjunctive version is very natural when the speaker is presenting the idea as something they had not yet realized.

Because poder ser means can be / could be.

The sentence is not simply saying that a rabbit was calm. It is expressing surprise about the possibility or capacity:

• ser tan tranquilo = to be so calm
• poder ser tan tranquilo = to be able to be so calm / to be capable of being so calm

So the speaker is surprised that a rabbit can have that quality in that context.

Ser is used for a quality or characteristic, while estar usually refers to a temporary state.

Here, ser tan tranquilo suggests that a rabbit can be calm in general, or that it can have a calm nature indoors.

If you said estar tan tranquilo, it would sound more like talking about a specific moment:

• El conejo está tranquilo = The rabbit is calm right now.
• Un conejo puede ser tranquilo = A rabbit can be calm by nature / in general.

So ser fits the idea better.

Tan means so, while muy means very.

• muy tranquilo = very calm
• tan tranquilo = so calm

In this sentence, tan sounds more natural because it expresses surprise at the degree. The speaker is basically saying: I didn’t know a rabbit could be that calm.

So tan highlights the unexpected level of calmness.

Because it agrees with un conejo, which is masculine singular.

Spanish adjectives must agree in gender and number with the noun they describe:

• un conejo tranquilo
• una coneja tranquila
• unos conejos tranquilos
• unas conejas tranquilas

So tranquilo matches conejo.

Because un conejo is being used in a general sense: a rabbit / any rabbit.

The sentence is not mainly about one specific rabbit already identified. It is more like saying something about rabbits as a type of animal, using a singular example.

• un conejo = a rabbit, rabbits in general
• el conejo = the rabbit, a specific rabbit, or sometimes the species in a more formal generic sense

Here, un conejo sounds very natural in everyday speech.

Que introduces the subordinate clause after sabía.

It works like that in English:

• No sabía que... = I didn’t know that...

So the structure is:

• No sabía = I didn’t know
• que un conejo pudiera ser tan tranquilo dentro de casa = that a rabbit could be so calm indoors

Both are possible, but they are not exactly the same.

• en casa = at home
• dentro de casa = inside the house / indoors

Dentro de casa puts more emphasis on being physically inside the home, often as opposed to outside.

So this sentence focuses on how calm a rabbit can be indoors, not just at home in a general sense.

Because Spanish often omits the article in set expressions referring to home:

• en casa
• dentro de casa
• volver a casa

In these cases, casa behaves a bit like home in English, which also often appears without an article.

If you said dentro de la casa, that would sound more like inside the house, referring to a specific building rather than the idea of home.

Yes. Pudiera and pudiese are both imperfect subjunctive forms of poder.

So these are both correct:

• No sabía que un conejo pudiera ser tan tranquilo...
• No sabía que un conejo pudiese ser tan tranquilo...

In modern everyday Spanish, especially in speech, the -ra form (pudiera) is usually more common.

Yes, but it would not mean exactly the same thing.

• pudiera ser tan tranquilo = could be so calm
• fuera tan tranquilo = was so calm / were so calm

With pudiera ser, the sentence focuses on possibility or capacity.
With fuera, it sounds more like a direct statement about a rabbit actually being that calm.

So the original sentence is better if the point is surprise about what rabbits are capable of being like indoors.