मेहमान मिठाई और जूस लाएँगे, कमरा सजाकर मोमबत्ती जलाएँगे।

Because the sentence is in the future tense, and the subject is understood as plural (or potentially honorific).

• लाएँगे = will bring
• जलाएँगे = will light

The ending -एँगे is a common masculine plural / honorific future ending.

Here, मेहमान is treated as plural, so the verbs are plural too.

In Hindi, मेहमान can be singular or plural depending on context.

• एक मेहमान आएगा = One guest will come
• मेहमान आएँगे = The guests will come

In your sentence, the verb लाएँगे shows that मेहमान is being understood as plural: guests.

Because Hindi often avoids repeating the subject when it stays the same.

So:

• मेहमान मिठाई और जूस लाएँगे
• (मेहमान) कमरा सजाकर मोमबत्ती जलाएँगे

The second मेहमान is omitted because it is already understood. English does this too sometimes:
The guests will bring sweets and juice, decorate the room, and light a candle.

सजाकर comes from सजाना = to decorate.

The form -कर often means having done X, after doing X, or by doing X.

So कमरा सजाकर means:

• after decorating the room
• having decorated the room
• sometimes more loosely, decorate the room and then...

In this sentence, it connects two actions:

1. decorate the room
2. light a candle

Both can be possible, but कमरा सजाना is very natural Hindi.

Hindi often uses a direct object without को, especially with inanimate things like कमरा.

So:

• कमरा सजाना = natural, common
• कमरे को सजाना = also possible, but often more marked or emphatic

English speakers often expect a marker like to or the room, but Hindi does not always need को for direct objects.

Hindi usually follows Subject–Object–Verb order, while English usually uses Subject–Verb–Object.

So:

• English: The guests will bring sweets and juice
• Hindi: मेहमान मिठाई और जूस लाएँगे

Literally, Hindi is closer to:

• Guests sweets and juice will bring

This is completely normal in Hindi.

Hindi does not always need a separate and between actions.

Here, the connection is shown by:

• the comma
• the -कर form in सजाकर

So the sentence naturally chains actions together:

• bring sweets and juice,
• decorate the room,
• light a candle

The और in the sentence only joins मिठाई and जूस.

Here और simply means and, joining two nouns:

• मिठाई और जूस = sweets and juice

It is not connecting the two larger parts of the sentence.
The larger actions are connected by the comma and by सजाकर.

Because the sentence is talking about a candle in a general or specific singular sense.

• मोमबत्ती जलाएँगे = will light a candle
• मोमबत्तियाँ जलाएँगे = will light candles

Hindi often leaves out articles like a and the, so मोमबत्ती can mean a candle or the candle, depending on context.

लाना means to bring.

Its future plural/honorific form is लाएँगे = will bring.

This verb is a little tricky because its stem changes. Learners often expect something simpler, but common forms are:

• मैं लाऊँगा / लाऊँगी = I will bring
• वह लाएगा / लाएगी = he/she will bring
• वे लाएँगे = they will bring

So लाएँगे is just the correct future form for they or you (honorific).

It comes from जलाना, which usually means to light or to cause to burn.

So:

• मोमबत्ती जलाना = to light a candle
• जलाएँगे = will light

This is different from जलना, which means to burn or to be lit on its own.

Compare:

• मोमबत्ती जलेगी = The candle will burn / be lit
• वे मोमबत्ती जलाएँगे = They will light the candle

That difference between जलना and जलाना is very common in Hindi:
one is intransitive, the other is causative/transitive.

Because ने is usually used with many perfective past transitive verbs, not with the simple future here.

This sentence is in the future, so you simply get:

• मेहमान मिठाई लाएँगे

not

• मेहमान ने मिठाई लाएँगे ✘

That is an important pattern in Hindi:

• past perfective often uses ने
• future does not

It is a very common loanword from English: juice → जूस.

Modern Hindi uses many everyday loanwords, especially for food, technology, and city life.

So in a sentence like this, जूस sounds completely natural.
A learner should not be surprised to see familiar English-origin words written in Devanagari.

Yes, that is close to the structure, but in natural English the smoother meaning is:

• The guests will bring sweets and juice, decorate the room, and light a candle.

The -कर form often has a slightly literal sense of after doing or having done, but in many real sentences it functions as a smooth connector between actions rather than a heavy formal sequence.

So both ideas are present:

• sequence of actions
• connected actions by the same subject