entyoukoodo ga areba, beddo no tikaku de sumaho wo zyuuden dekiru.

あれば is the conditional form of ある.

• ある = to exist / to be available / to have
• あれば = if there is, if you have, or if one is available

So 延長コードがあれば means if there is an extension cord or if you have an extension cord.

Because ある normally marks the thing that exists with が.

• 延長コードがある = there is an extension cord

So in the conditional:

• 延長コードがあれば = if there is an extension cord

Using は is possible in some contexts, but it would add contrast or a special emphasis, like as for an extension cord.... The neutral, natural form here is が.

It can feel like either one in English.

Japanese often uses ある for the existence or availability of inanimate things. So:

• 延長コードがあれば can mean if there is an extension cord
• or more naturally in some situations, if you have an extension cord

The Japanese does not need to choose as strictly as English does.

近く means nearby area, vicinity, or simply near.

In ベッドの近く:

• ベッド = bed
• の = links one noun to another
• 近く = vicinity / near area

So ベッドの近く literally means the area near the bed, or more naturally, near the bed.

Because の connects nouns.

Here it works like English of or an attributive link:

• ベッドの近く = the bed’s vicinity = near the bed

This is a very common pattern in Japanese:

• 学校の前 = in front of the school
• 駅の近く = near the station
• 家の中 = inside the house

Because で marks the place where an action happens.

Here, the action is charging the smartphone, so:

• ベッドの近くでスマホを充電できる
• = you can charge your smartphone near the bed

A simple way to think about it:

• で = where an action takes place
• に = destination, target location, or point of existence in many cases

Since charging is an action done at that location, で is the natural choice.

This is a very common learner question.

It is true that できる often goes with が:

• 日本語ができる = can do/speak Japanese

But here the structure is based on the verbal expression スマホを充電する:

• スマホを充電する = charge a smartphone

When this becomes potential, Japanese very often keeps the original object marker:

• スマホを充電できる = can charge a smartphone

So を is very natural here.

You may also sometimes see が with potential expressions in Japanese, but with a verb like 充電する, スマホを充電できる is a normal and natural way to say it.

It functions like one verbal expression, but it is built from two parts:

• 充電 = charging
• できる = can do / is possible

Together:

• 充電できる = can charge

You can think of it as the potential form of 充電する.

Related forms:

• 充電する = to charge
• 充電できる = can charge
• 充電できます = can charge (polite)

The subject is omitted.

Japanese often leaves out subjects when they are obvious from context. So this sentence could mean:

• I can charge my phone near the bed
• You can charge your phone near the bed
• One can charge a smartphone near the bed

The sentence focuses on the condition and the possibility, not on explicitly naming the subject.

Because Japanese very often puts the condition before the result:

• if X, Y

So:

• 延長コードがあれば、ベッドの近くでスマホを充電できる。
• If there is an extension cord, you can charge your smartphone near the bed.

This order is extremely natural in Japanese.

The comma after あれば just marks a pause between the condition and the main statement.

You could, but the nuance changes.

• あれば = if there is / if you have one
  
  • neutral and very natural here
• あるなら = if there is one / if that’s the case
  
  • often sounds like it depends on prior information or what someone just said
• あると = when/if there is one
  
  • often sounds more general, automatic, or habitual

For a practical statement like this, あれば is a very good choice.

It is plain style.

• 充電できる = plain
• 充電できます = polite

So a polite version would be:

• 延長コードがあれば、ベッドの近くでスマホを充電できます。

Both are correct; the choice depends on the situation.