péngyou yào yí gè shuāngrénjiān.

This is because of a common tone-change rule for 一.

• 一 is normally yī when said by itself or when counting.
• Before a 4th-tone syllable, it changes to yí.
• Before a 1st-, 2nd-, or 3rd-tone syllable, it usually changes to yì.

Here it comes before 个 gè, which is 4th tone, so 一个 is pronounced yí gè.

个 is a measure word or classifier.

In Mandarin, when you count nouns, you usually say:

number + classifier + noun

So:

• 一个双人间 = one double room

English learners often want to say just one room, but Mandarin normally needs that extra classifier word in between.

It is understandable, but for hotel rooms, 间 is usually more natural than 个.

So native speakers often say:

• 一间双人间

instead of:

• 一个双人间

Why?

• 间 is the usual classifier for rooms.
• 个 is the very general classifier, so beginners learn it early and use it a lot.

So the sentence is easy to understand, but 一间双人间 sounds more idiomatic in many real-life situations.

Mandarin word order here is very straightforward:

subject + verb + object

So:

• 朋友 = friend
• 要 = wants / would like
• 一个双人间 = one double room

That gives:

朋友 要 一个双人间

The object stays together as one chunk: number + classifier + noun.

Mandarin does not use articles like a, an, or the.

So 朋友 can mean different things depending on context, such as:

• a friend
• the friend
• my friend
• sometimes even friends

The surrounding situation tells you which meaning is intended.

That is very normal in Chinese, even though it feels incomplete to English speakers at first.

By itself, 朋友 can be either singular or plural. Mandarin nouns usually do not change form for plural the way English nouns do.

In this sentence, it most likely means friend in the singular, because that fits the sentence naturally: one friend wants one double room.

If you wanted to make the plural more explicit, you might say:

• 朋友们 = friends

But 们 is not always necessary, and native speakers often rely on context instead.

要 here means something like:

• want
• need
• would like

In practical situations such as ordering, buying, or booking, 要 is very common.

So at a hotel desk, 要一间双人间 can mean:

• want a double room
• would like a double room

It can sound a little direct if said with the wrong tone of voice, but in many service situations it is completely normal.

If you want to sound a bit softer or more explicit, you could also say:

• 想要一间双人间 = would like a double room
• 想订一间双人间 = would like to book a double room

It literally breaks down like this:

• 双 = double / two
• 人 = person
• 间 = room

So 双人间 literally means two-person room, which is the usual way to say double room.

A related word is:

• 单人间 = single room

Because the second syllable is usually pronounced with a neutral tone in normal speech.

So:

• 朋 = péng
• 友 = you (light, neutral tone)

This is very common in everyday Mandarin. Some two-syllable words have a strong first syllable and a lighter second syllable.

So 朋友 sounds more like:

• péngyou

rather than two equally stressed syllables.

Yes, it could, depending on context.

Mandarin often leaves out possessive words like my when they are obvious from the situation. So 朋友 might mean:

• a friend
• my friend
• the friend

If the speaker wants to make my explicit, they can say:

• 我的朋友要一间双人间。

But in conversation, Mandarin often prefers the shorter version when the meaning is already clear.