S jedne strane pogodnost mi pomaže, s druge strane odgovornost smeta.

Yes. It is a very common pairing meaning on the one hand ... on the other hand.

Literally, it is from one side ... from the other side, but in normal usage it introduces two contrasting perspectives, advantages, or factors.

So in this sentence, it sets up a contrast:

• one factor is helpful
• another factor is a problem

They agree with strane, which comes from strana and is here in the genitive singular.

Breakdown:

• strana = side
• jedna strana = one side
• druga strana = the other side

But after s in this idiomatic expression, Croatian uses the genitive:

• s jedne strane
• s druge strane

Because strana is feminine singular, the modifiers also have to be feminine singular genitive:

• jedne
• druge

Mi means to me and is the dative singular form of ja.

The verb pomagati / pomoći takes the person being helped in the dative:

• To mi pomaže. = That helps me.
• Pogodnost mi pomaže. = The convenience/advantage helps me.

So mi is not optional if you want to say who is being helped.

Because mi is a clitic, an unstressed short word. In Croatian, clitics usually go in the second position of their clause.

Here, S jedne strane works like an introductory expression set off by a comma. The main clause is:

• pogodnost mi pomaže

Within that clause, mi comes after the first stressed element, which is pogodnost.

That is why pogodnost mi pomaže sounds natural.

That is a very reasonable question.

Yes, smetati often also takes the dative:

• To mi smeta. = That bothers me / gets in my way.

So if the intended meaning is specifically responsibility bothers me, then odgovornost mi smeta would be very natural and more parallel with the first half.

Without mi, odgovornost smeta sounds more general:

• responsibility is a nuisance
• responsibility is an obstacle
• responsibility causes problems

So both are possible, but they are slightly different in feel.

They are in the nominative singular because they are the subjects of the verbs:

• pogodnost ... pomaže
• odgovornost ... smeta

Both nouns are feminine nouns ending in -ost, and they often look the same in nominative and accusative singular. Here, their role in the sentence tells you they are nominative.

Smetati can mean several related things, such as:

• to bother
• to disturb
• to be in the way
• to interfere
• to be a hindrance

So in this sentence, odgovornost smeta does not have to mean emotional annoyance only. It can also mean that responsibility creates difficulty or gets in the way.

That is why English translations may vary depending on context.

Because each verb has a singular subject:

• pogodnost = singular
• odgovornost = singular

So Croatian uses 3rd person singular present tense:

• pomaže = helps
• smeta = bothers / interferes / is a nuisance

This present tense can describe a general situation, not only something happening right now.

Very often, yes.

Croatian frequently uses:

• S jedne strane ..., a s druge strane ...

The conjunction a helps mark contrast and can make the sentence feel smoother or more explicitly balanced.

So this version is very natural:

• S jedne strane pogodnost mi pomaže, a s druge strane odgovornost mi smeta.

But the version without a is still understandable, especially because the parallel structure already makes the contrast clear.