konsento ga hitotu sika nai heya ha, entyoukoodo ga nai to huben desu.

It breaks down like this:

• コンセントが一つしかない部屋 = a room that has only one outlet
• は = marks that room as the topic
• 延長コードがないと = if there is no extension cord
• 不便です = it is inconvenient

So the sentence is:

• [A room that has only one outlet]
  • is inconvenient
    • if there is no extension cord

A very natural English rearrangement is:

• A room with only one outlet is inconvenient without an extension cord.

This is a relative clause, which in Japanese comes before the noun it modifies.

So:

• コンセントが一つしかない = has only one outlet
• 部屋 = room

Put together:

• コンセントが一つしかない部屋 = a room that has only one outlet

Japanese does not need words like that, which, or who here.
The verb phrase simply goes before the noun.

Compare:

• 本を読む人 = a person who reads books
• 日本にある店 = a store that is in Japan
• コンセントが一つしかない部屋 = a room that has only one outlet

しか is a very common pattern meaning only, but it must be used with a negative form.

So:

• 一つしかない = there is only one
• literally, something like there isn’t more than one

This is different from English, because English uses only without needing a negative verb.

Examples:

• お金が千円しかない。 = I only have 1,000 yen.
• 一人しか来なかった。 = Only one person came.
• コンセントが一つしかない。 = There is only one outlet.

So in your sentence, 一つしかない is the standard way to say there is only one.

They do different jobs.

1. 一つしかない

Here, ない is part of the しか～ない pattern:

• 一つしかない = there is only one

2. 延長コードがないと

Here, ない is just the ordinary negative of ある:

• 延長コードがない = there is no extension cord
• 延長コードがないと = if there is no extension cord

So even though both are ない, they are functioning differently.

Because コンセント is the thing that exists / does not exist in that clause.

In:

• コンセントが一つしかない

the outlet is the subject of ない in the sense of existence:

• There is only one outlet

With ある / ない, the thing that exists or does not exist is often marked by が.

So this is very natural:

• コンセントが一つしかない

rather than treating コンセント as the topic.

Because 部屋 is the topic of the main sentence.

The structure is:

• コンセントが一つしかない部屋 = room that has only one outlet
• は = as for...
• 延長コードがないと不便です = it is inconvenient if there is no extension cord

So the sentence is saying:

• As for a room that has only one outlet, it is inconvenient if there is no extension cord.

Inside the relative clause, コンセント is marked with が.
Outside that clause, the whole noun phrase コンセントが一つしかない部屋 becomes the topic, so it takes は.

Here, と is a conditional marker meaning if or when.

So:

• 延長コードがないと = if there is no extension cord

In this sentence, it expresses a fairly natural consequence:

• If there is no extension cord, it is inconvenient.

This と is often used when the result is:

• automatic
• habitual
• generally true
• a natural consequence

Examples:

• 春になると、暖かくなる。 = When spring comes, it gets warm.
• このボタンを押すと、ドアが開きます。 = If you press this button, the door opens.
• 延長コードがないと、不便です。 = If there is no extension cord, it is inconvenient.

Yes. A few alternatives are possible, but the nuance changes slightly.

Your sentence:

• 延長コードがないと不便です。
• very natural for a general consequence

Also possible:

• 延長コードがなければ不便です。
• also means it is inconvenient if there is no extension cord

• sounds a little more formal or neutral

• 延長コードがないなら不便です。
• sounds more like if that’s the case / if there is no extension cord, then it’s inconvenient
• more situational or contrastive

For this kind of general statement, ないと or なければ both work well, but ないと feels especially natural in everyday speech.

Japanese often uses 〜つ as a general-purpose counter for small numbers, especially in everyday speech.

So:

• 一つ = one
• 二つ = two
• 三つ = three

For something like an outlet, 一つ is completely natural.

You might also hear other counting styles in some contexts, but 一つ is simple and normal here.

So:

• コンセントが一つしかない = there is only one outlet

is a very ordinary way to say it.

Yes. 不便 means inconvenient and is a na-adjective.

So:

• 不便です = is inconvenient (polite)
• 不便だ = is inconvenient (plain)

Examples:

• この場所は不便です。 = This place is inconvenient.
• 駅から遠くて不便だ。 = It’s inconvenient because it’s far from the station.

In your sentence:

• 延長コードがないと不便です。 = Without an extension cord, it is inconvenient.

Not exactly in the same way.

一つしかない

This means:

• there is only one
• strongly tied to the idea that the number is limited

一つだけ

This also means only one, but it works differently.

For example:

• コンセントが一つだけある部屋
• a room that has just one outlet

That is possible, but it is not the same structure as your sentence.

You normally would not say:

• コンセントが一つだけない部屋

because that would mean something different and sound odd.

So in your sentence, しか～ない is the most natural pattern for has only one outlet.

It can be understood as a general statement.

Japanese often uses a topic like this to talk about a type of thing in general:

• コンセントが一つしかない部屋は、延長コードがないと不便です。

This means something like:

• Rooms with only one outlet are inconvenient without an extension cord.

It does not have to mean one specific room unless the context makes that clear.

So even though 部屋 is singular in form, Japanese often leaves number flexible, and English may translate it as either:

• A room with only one outlet...
• Rooms with only one outlet...

depending on context.