Learn Hardcore Japanese: sono huuhu ha seikaku ga tigatte mo, otagai wo sonkeisite imasu. - Even though the couple’s personalities are different, they respect each other.

If you said その夫婦が, it would sound more like you’re focusing on them as the one(s) who do the action in contrast to others (subject focus), e.g., “it is that couple who (unlike others) respect each other.” In this context, the neutral, natural choice is は to introduce them as the topic.

Why is 性格 followed by が and not は? Could I say 性格は違っても?

が here marks 性格 (personality) as the thing that is different.

性格が違っても literally: even if (their) personalities are different.

You can say 性格は違っても. The nuance changes slightly:

性格が違っても: stating the fact that their personalities differ.
性格は違っても: slightly contrasts personality with other aspects, like “as for their personalities, they’re different (but still…).”

Both are grammatically correct; 性格が違っても is just a straightforward description.

What is the function of 〜ても in 違っても? How is it different from でも?

〜ても is a grammar pattern: (verb/adjective in て-form) + も = “even if / even though”.

違っても = 違う (to be different) in て-form (違って) + も
So 性格が違っても = even if their personalities are different / even though their personalities are different.

でも (written separately) is a conjunction like “but / however / even though” and usually follows a full clause:

性格が違います。でも、お互いを尊敬しています。
Their personalities are different. But they respect each other.

Using 違っても makes it one smooth sentence instead of two sentences joined by でも.

Why is it 違って and not something like 違くて? Is 違う an adjective?

違う is a verb, not an i-adjective.

Dictionary form: 違う = to be different, to differ.
て-form of a verb ending in う is: 〜って → 違って.

So:

違う → 違って (verb)
Then add も: 違っても (even if [they] are different).

Forms like 違くて are colloquial and non-standard; the correct textbook form is 違って.

What does お互い (おたがい) mean exactly? Is it a pronoun, like “each other”?

お互い means “each other / one another / mutually” and functions mostly as a noun-like word.

In this sentence:

お互いを尊敬しています
Literally: [they] respect each other.

Here:

お互い = “each other”
を marks お互い as the direct object of 尊敬する (to respect).

So structurally it’s like saying: “They respect each other (as an object).”

Why is it お互いを尊敬しています and not お互いに尊敬しています? What’s the difference between お互いを and お互いに?

Both exist, but they’re used a bit differently.

お互いを尊敬しています
- お互い is the object (“each other”)
- Literal: They respect each other.
- Focus is on the action toward the other person.
お互いに尊敬しています is a bit awkward; you’d more naturally say:
- お互いに尊敬し合っています
- Here お互いに = “mutually,” an adverb-like use.
- 尊敬し合う = “to respect each other, to respect one another.”

Common patterns:

お互いを尊敬しています (object use)
お互いに尊敬し合っています (adverb + reciprocal verb)

In your sentence, お互いを尊敬しています is perfectly natural and common.

Why is it 尊敬しています (progressive form) instead of just 尊敬します? Does it mean they’re respecting each other right now?

In Japanese, 〜している often expresses a continuing or habitual state, not just “doing right now.”

尊敬する = to respect
尊敬している / 尊敬しています = to respect (as an ongoing attitude/state)

So 尊敬しています here means:

they (habitually/continuously) respect each other, not
they are right now in the middle of the act of respecting.

If you said 尊敬します, it would sound more like a one-time or future action: they will/shall respect each other (on some occasion), which is not the intended meaning here.

Could I say 尊敬し合っています instead of お互いを尊敬しています? Is there a nuance difference?

Yes, you can.

尊敬し合う = “to respect each other / to respect one another.”
尊敬し合っています = they respect each other (mutual action built into the verb).

Nuance:

お互いを尊敬しています
Explicitly uses お互い (“each other”). Very clear and natural.
尊敬し合っています
Uses a reciprocal verb; sounds slightly more “constructed” but is also natural.

You can even combine them for emphasis:

お互いを尊敬し合っています
(a bit emphatic, “mutually respect each other.”)

In everyday usage, お互いを尊敬しています is simple and clear.

What is the politeness level of 尊敬しています? How would it look in plain/casual form?

尊敬しています is polite form (です・ます style).

Plain (casual) forms:

尊敬している (present-state)
Full casual sentence:
その夫婦は性格が違っても、お互いを尊敬している。

So:

Polite: 尊敬しています
Casual/plain: 尊敬している

Can the word order in this sentence be changed? For example, can I move お互い?

Japanese word order is relatively flexible, especially inside the clause, as long as you keep the verb at the end.

Your original:

その夫婦は性格が違っても、お互いを尊敬しています。

Possible variants (all grammatical, slightly different emphasis):

その夫婦は、お互いを尊敬していますが、性格が違っています。
その夫婦は、お互いを尊敬していて、性格が違っています。

However, within the original structure, you’d normally keep:

性格が違っても together as one chunk
and お互いを尊敬しています as another.

Putting お互い before 性格が違っても would be strange; お互いを belongs right before 尊敬しています as its object.

Is there any difference between this sentence and one using けれども, like
その夫婦は性格が違うけれども、お互いを尊敬しています。?

They’re very close in meaning.

性格が違っても = even if / even though their personalities are different.
性格が違うけれども = although / though their personalities are different.

Nuance:

〜ても often feels a bit more like “even if (that condition holds).”
〜けれども is a more straightforward “although / but.”

In this specific context, the difference in nuance is small; both are natural ways to say:

Although their personalities are different, they respect each other.

Does 性格 refer to one shared personality or each person’s personality? How is number handled here?

Japanese often does not mark singular/plural explicitly. Here:

性格が違っても is understood from context as
even if their personalities are different (from each other).

So 性格 here implicitly refers to each spouse’s personality.
The plural idea (“personalities”) is carried by the context of 夫婦 (two people), not by a special plural form of 性格.