Перед приходом гостей мы поставили свечи, надули шарики и только потом сели пить чай.

Because the preposition перед normally requires the instrumental case when it means before in time or in front of in space.

So:

• приход = arrival
• приходом = instrumental singular of приход

In this sentence, перед приходом гостей means before the guests’ arrival.

Гостей is in the genitive plural because it depends on the noun приход and answers the idea arrival of whom?

So:

• приход гостей = the arrival of the guests
• перед приходом гостей = before the arrival of the guests

This is a very common Russian pattern: a noun followed by another noun in the genitive to show possession or relation.

Examples:

• начало фильма = the beginning of the film
• возвращение брата = the brother’s return
• приход гостей = the guests’ arrival

Because приход refers to one event, not to the number of people.

Even if many guests are coming, their coming can still be treated as one single event: the arrival.

So:

• приходом = one arrival event
• гостей = of the guests

English works the same way in phrases like before the guests’ arrival, where arrival is singular.

Поставили is the past tense of поставить, the perfective verb meaning to put, to place, or to set up.

With свечи, it usually means they set out / put up the candles somewhere, for example on a table or in a room.

Important nuance: поставили свечи does not automatically mean they lit them.

If you want to say lit the candles, Russian would usually say:

• зажгли свечи

So here the meaning is more like they placed the candles as part of the preparations.

Because this sentence describes a sequence of completed actions, and Russian typically uses the perfective aspect for that.

• надули = perfective past, blew up / inflated
• надували = imperfective past, were inflating / used to inflate

In this sentence, the idea is:

1. they put out the candles,
2. they inflated the balloons,
3. only then they sat down for tea.

Each action is presented as finished, so perfective verbs fit naturally:

• поставили
• надули
• сели

Шарики is the plural of шарик, which is a diminutive form of шар.

In everyday Russian, шарик / шарики very often means balloon / balloons, especially party balloons.

The diminutive gives it a more natural, everyday feel here. Since the sentence is about guests, candles, and tea, шарики strongly suggests decorative balloons for a celebration.

So while шары can mean spheres / balls / balloons in some contexts, шарики is the more typical word here.

Только потом means only then or only after that.

It adds emphasis: they did not sit down for tea immediately. First they finished the preparations, and only after that did they sit down.

So it highlights the order of events more strongly than just потом.

Compare:

• потом сели пить чай = then they sat down to drink tea
• только потом сели пить чай = only then did they sit down to drink tea

Because сесть + infinitive is a common Russian structure meaning to sit down to do something or more generally to begin doing something after settling down.

So:

• сели пить чай = sat down to drink tea / sat down for tea
• пили чай = drank tea / were drinking tea

The version with сели gives an extra sense of transition: after the preparations, they settled down and started having tea.

This is very natural Russian:

• сели ужинать = sat down to have dinner
• сел читать = sat down to read
• сели обсуждать = sat down to discuss

Because after пить, the noun goes in the accusative case.

For чай, the accusative singular is the same as the nominative singular because it is an inanimate masculine noun:

• nominative: чай
• accusative: чай

So пить чай is grammatically normal and also a very common expression meaning to drink tea / have tea.

Because the subject is мы = we.

In Russian past tense, verbs agree with the subject in number and, in the singular, also in gender. In the plural, there is just one plural form:

• я поставил / поставила
• мы поставили

So:

• мы поставили
• мы надули
• мы сели

Also, Russian past tense does not show person the way present tense does. The ending here tells you the action is past plural, and мы gives the subject clearly.

The word order here is very natural and neutral for narration:

Перед приходом гостей — time frame first
мы — subject
поставили свечи, надули шарики — completed preparations
и только потом сели пить чай — final action, with emphasis on order

Russian word order is flexible, so parts of the sentence could be rearranged, but the given version sounds smooth and idiomatic.

Placing только потом before сели helps emphasize that the tea happened strictly after the earlier actions were finished.