Потом начался короткий диалог, и одна реплика заставила весь зал засмеяться.

Because the verb here is начаться, not начать.

• начать = to begin/start something
• начаться = to begin/start on its own

So:

• начался короткий диалог = a short dialogue began
• If you used начал, you would need a subject who started something, for example: Он начал диалог = He started the dialogue

Also, диалог is masculine singular, so the past tense form is начал + -ся → начался.

-ся often makes a verb reflexive, but in many cases it is better understood more broadly as changing the verb’s meaning.

Here, it turns начать into начаться:

• начать что-то = to start something
• начаться = to start, to begin

So диалог начался literally means something like the dialogue started itself, but in natural English we simply say the dialogue began.

Because короткий agrees with диалог in gender, number, and case.

• диалог is masculine
• singular
• nominative case, because it is the subject of начался

So the adjective must also be masculine singular nominative:

• masculine nominative: короткий
• feminine nominative: короткая
• neuter nominative: короткое
• plural nominative: короткие

That is why it is короткий диалог.

Because реплика is a feminine noun.

Russian numbers one change for gender:

• один for masculine
• одна for feminine
• одно for neuter

So:

• один диалог
• одна реплика
• одно слово

Since реплика is feminine, the correct form is одна.

In this sentence, реплика means something like:

• a remark
• a line
• an utterance
• a brief comment

It is often used for a short spoken contribution in a conversation, dialogue, play, or interview.

So одна реплика заставила весь зал засмеяться means that one particular line or remark made everyone laugh.

Because the subject is одна реплика, and реплика is feminine singular.

In the past tense, Russian verbs agree with the subject in gender and number:

• masculine: заставил
• feminine: заставила
• neuter: заставило
• plural: заставили

So:

• реплика заставила
• диалог заставил would be masculine
• слово заставило would be neuter

Here, реплика is feminine, so the verb is заставила.

Весь means all, whole, or entire, and it agrees with зал.

• зал = hall, auditorium, or by extension the audience
• весь зал = the whole hall / the entire audience

In context, it usually means that everyone in the hall laughed.

Grammatically, весь зал is the direct object of заставила, so it is in the accusative case. But because зал is masculine inanimate, its accusative form looks the same as the nominative:

• nominative: весь зал
• accusative: весь зал

So it looks unchanged, even though it is functioning as the object.

Because засмеяться means to burst out laughing or to start laughing, while смеяться means to laugh in a more general or ongoing sense.

After заставить, Russian often uses an infinitive to show what someone was made to do. Here the idea is not just that the audience laughed in general, but that the remark triggered the start of laughter.

So:

• заставила смеяться = made them laugh / kept them laughing
• заставила засмеяться = made them start laughing, made them burst out laughing

In this sentence, засмеяться fits better because the remark caused an immediate reaction.

Yes. Начаться is a perfective verb in this form.

Russian aspect matters a lot:

• начинаться = imperfective, focusing on process or repeated action
• начаться = perfective, focusing on the event as a complete occurrence

Here, Потом начался короткий диалог presents the dialogue as a single event that began at a particular moment: Then a short dialogue began.

If you used начинался, it would suggest more of a process, background description, or repeated situation, which is not the main idea here.

Because и is joining two separate clauses, each with its own subject and verb:

1. Потом начался короткий диалог
2. одна реплика заставила весь зал засмеяться

Each part can stand as its own clause, so Russian normally puts a comma before и in this situation.

Yes. Russian word order is more flexible than English word order, although the original sentence is very natural.

For example, you could also say:

• Потом короткий диалог начался, и одна реплика заставила весь зал засмеяться.
• И потом начался короткий диалог, и одна реплика заставила весь зал засмеяться.
• Одна реплика заставила засмеяться весь зал.

But the original version sounds smooth and neutral:

• Потом начался короткий диалог, и одна реплика заставила весь зал засмеяться.

Russian often uses word order to manage emphasis, not just grammar. In the original, the flow is very natural: first the dialogue begins, then one remark causes the laugh.

It can mean both, depending on context.

Literally:

• зал = hall, auditorium, large room

But in many sentences, it refers to the people there by extension:

• весь зал засмеялся = the whole audience laughed

So in this sentence, весь зал most naturally means everyone in the hall/the whole audience, not just the physical room.

Потом means then, after that, or later.

It connects this sentence to previous events in the story. It tells you that first something else happened, and after that:

• начался короткий диалог = a short dialogue began

So потом helps move the narrative forward. It is a very common storytelling word in Russian.