Learn Hardcore Swedish: Efter konserten är bussen full av glad publik. - After the concert the bus is full of happy audience members.

Using efter konsert without an article would sound incomplete or unidiomatic in this context. You could see efter konsert only in very special, almost headline‑like or shorthand styles, but not in normal sentences.

Why does the verb come right after efter konserten? Why not Efter konserten bussen är…?

Swedish has the V2 word order rule in main clauses: the finite verb must be in second position, no matter what comes first.

In the neutral order:

Bussen är full av glad publik efter konserten.
(Subject bussen is first, verb är is second.)

If you move something else to the front (like efter konserten), the verb still has to be second:

Efter konserten är bussen full av glad publik.
(Adverbial efter konserten is first, verb är is second, subject bussen is third.)

Efter konserten bussen är … breaks the V2 rule, so it’s ungrammatical.

Why is it bussen and not just buss?

Again, it’s about definiteness.

en buss = a bus
bussen = the bus

In this sentence, we are talking about a specific bus (for example, the bus that leaves from the venue). In English you also say “the bus” in this context, and Swedish matches that with bussen.

If you wanted to talk more generally, you could say:

Efter konserter är bussar ofta fulla.
“After concerts, buses are often full.” (general statement)

Why is the verb in the present tense (är) and not past (var) when this seems to describe something that happens after a concert?

With är (present), the sentence sounds like a general statement about what typically happens:

Efter konserten är bussen full av glad publik.
= “(Usually) after the concert, the bus is full of happy audience.”

If you want to refer to one specific past concert, you’d use the past:

Efter konserten var bussen full av glad publik.
= “After the concert, the bus was full of happy audience.”

So:

är → general, habitual, or a vivid “commentary” style.
var → a specific past situation.

Why is it full and not fullt or fulla?

This is about adjective agreement in predicative position (after är, blir, känns, etc.).

For en-words, singular in this kind of structure, the adjective takes its base form:

Bussen är full. (en buss → singular → full)
Bilen är röd. (en bil → röd)

For plural, you use -a:

Bussarna är fulla.
Bilarna är röda.

For ett-words in singular when there is a formal det:

Det är fullt här.
Rummet är fullt.

So bussen is an en-word, singular, therefore full is correct.

Why is it full av and not full med?

Both full av and full med exist, and in many cases they overlap, but:

full av often sounds a bit more literal or descriptive, like “filled with / full of (something that occupies the space)”.
full med can sound a bit more casual, and is very common in speech.

In this sentence, full av glad publik is very natural: the bus is literally “full of” people.

You could also say:

Bussen är full med glada människor.

That’s also idiomatic. In this exact phrase, full av glad publik might feel slightly more neutral or “written” style than full med.

Why is it glad publik and not glada publik?

Here, publik is singular:

en publik (a crowd / an audience)
publiken (the audience)

For singular en-words in attributive position (before the noun), the adjective takes the base form:

en glad publik
en stor publik
en entusiastisk publik

Glada is used for plural or for definite forms:

glada människor (happy people)
den glada publiken (the happy audience)

So:

glad publik is correct (indefinite singular).
glada publik is wrong.
den glada publiken would be correct if you make it definite.

Why is it publik and not publiken, even though we seem to be talking about “the audience” from that concert?

Swedish often uses indefinite singular mass nouns to refer to a kind of group or substance in a more general way.

publik can mean “audience” in a generic or mass sense.
publiken is “the (specific) audience”.

Both are possible here, but with slightly different nuances:

…full av glad publik.
Emphasizes “full of happy audience (people),” more like a general mass of audience, not focusing so much on which audience.
…full av den glada publiken.
Very specifically “full of the happy audience (that was at the concert).” Sounds a bit heavier, more explicit.

In everyday language, full av glad publik sounds light and natural; the context (after the concert) already tells us which audience it probably is.

Is publik treated as singular or plural in Swedish?

Grammatically, publik is a singular en-word:

en publik
publiken

Verbs and adjectives agree as if it were singular:

Publiken var mycket nöjd.
“The audience was very satisfied.”

There is a plural publiker, but it is relatively rare and usually means different audiences in different situations:

Artisten har uppträtt för många olika publiker.
“The artist has performed for many different audiences.”

So in glad publik, the adjective agrees with singular publik, not with a plural form.

Could I say glada människor instead of glad publik? Would it change the meaning?

Yes, you can say:

Efter konserten är bussen full av glada människor.

This is completely correct but slightly different in nuance:

glad publik = emphasizes them as an audience, people who have just been in the role of listeners/spectators.
glada människor = emphasizes them simply as people who are happy, with no special role.

Both sentences are natural; the original just highlights that they are the concert audience.

What is the difference between efter and före in this kind of sentence?

They’re basic time prepositions:

efter konserten = after the concert
före konserten = before the concert

Examples:

Före konserten är bussen nästan tom.
“Before the concert, the bus is almost empty.”
Efter konserten är bussen full av glad publik.
“After the concert, the bus is full of happy audience.”

How would the sentence change if I wanted to focus on the bus instead of on “after the concert”?

You can change the word order to put bussen first, but because of the V2 rule, the verb är must still be in second position:

Bussen är full av glad publik efter konserten.

This version:

still means the same thing
sounds a bit more neutral, with the subject first, like in English.

Both:

Efter konserten är bussen full av glad publik.
Bussen är full av glad publik efter konserten.

are correct; the difference is just which part you highlight first.