Davno sam se učlanio u biblioteku.

Davno means a long time ago or long ago.

In this sentence, it tells you that the action happened far back in the past. It is an adverb of time.

• Davno sam se učlanio u biblioteku. = I joined the library a long time ago.

A useful contrast:

• skoro = recently
• davno = long ago

Sam is the 1st person singular form of the auxiliary verb biti (to be) used to form the Serbian past tense.

So:

• sam = I have / I did as part of the past-tense structure
• učlanio = past active participle

Together:

• sam učlanio = I joined

In Serbian, the past tense is commonly built like this:

• auxiliary + past participle

Examples:

• Ja sam došao. = I came.
• Ja sam radio. = I worked.
• Ja sam se učlanio. = I joined / became a member.

This is because sam is a clitic. Serbian clitics usually go in the second position in the clause.

So in:

• Davno sam se učlanio u biblioteku.

the first element is davno, and then the clitics come:

• sam
• se

That is why sam appears there.

You could also say:

• Ja sam se davno učlanio u biblioteku.
• Davno sam se učlanio u biblioteku.

Both are possible, but clitic placement follows Serbian word-order rules, not English logic.

Se is a reflexive particle that belongs to the verb učlaniti se.

The verb is not normally used here without se. The full dictionary form is:

• učlaniti se = to become a member, to join

So you should learn it as a unit:

• učlaniti se u nešto = to join something

Examples:

• Učlanio sam se u teretanu. = I joined the gym.
• Učlanila se u klub. = She joined the club.

Even though se is called reflexive, in many verbs it does not translate literally as oneself. It is just part of the verb.

It is the Serbian past tense (often called perfekat).

Structure:

• sam = auxiliary
• se = reflexive particle
• učlanio = past participle

So:

• sam se učlanio = I joined / I became a member

In everyday Serbian, this is the normal way to talk about completed actions in the past.

Učlanio agrees with the speaker’s gender and number.

Here, učlanio shows that the speaker is:

• singular
• masculine

A female speaker would say:

• Davno sam se učlanila u biblioteku.

Other forms:

• učlanio sam se = I joined (male speaker)
• učlanila sam se = I joined (female speaker)
• učlanili smo se = we joined (mixed group or all male)
• učlanile smo se = we joined (all female)

This is a very important feature of Serbian past tense.

Because after učlaniti se, Serbian normally uses u + accusative to show the organization or group you are joining.

So:

• u biblioteku = into the library / as a member of the library

This is not about physically being located inside the building. It is the standard construction with this verb.

Compare:

• učlaniti se u biblioteku = to join the library
• biti u biblioteci = to be in the library
• raditi u biblioteci = to work in the library

So:

• u biblioteku = direction / entry into membership
• u biblioteci = location inside the library

Here it means the institution or library as an organization, not just the physical building.

Because the verb učlaniti se means to become a member, the sentence is about getting a library membership.

So this is like:

• joining the library system
• becoming a registered library member

Not:

• entering the building
• going to the library

Yes, učlaniti se is perfective. It describes a completed action: the moment or event of becoming a member.

That fits this sentence well, because joining a library is usually seen as a single completed act.

A related imperfective verb is:

• učlanjivati se

But that is much less natural here unless you are talking about repeated attempts, a process, or habitual action.

So:

• učlanio sam se = I joined / I became a member
• učlanjivao sam se = I was joining / I used to join / I kept joining

In this sentence, the perfective form is the normal choice.

No. That would be incorrect here.

You need se because the verb is učlaniti se. Without se, you do not have the normal verb meaning to become a member.

So the correct form is:

• Davno sam se učlanio u biblioteku.

A good habit is to memorize reflexive verbs together with se, for example:

• nadati se
• sećati se
• učlaniti se
• smejati se

Yes, Serbian word order is fairly flexible, but not completely free. The meaning stays similar, while the emphasis changes.

For example:

• Davno sam se učlanio u biblioteku.
  Neutral; emphasis on long ago.

• U biblioteku sam se učlanio davno.
  More emphasis on the library or a contrast with something else.

• Ja sam se davno učlanio u biblioteku.
  The subject ja is expressed for emphasis.

What usually stays important is the placement of clitics like sam and se, which tend to appear near the beginning in the proper clitic position.

Yes. A common alternative is:

• Davno sam postao član biblioteke.

This literally means I became a member of the library a long time ago.

Both are natural, but učlaniti se u biblioteku is very common and compact.

Compare:

• učlaniti se u biblioteku = to join the library
• postati član biblioteke = to become a member of the library

Both are correct; the original sentence is slightly more direct and idiomatic for everyday speech.