siharaikigen wo kakuninsuru.

It is read:

しはらいきげん を かくにんする

• 支払期限 = しはらいきげん
• を = o
• 確認する = かくにんする

A small note: even though を is written with the kana を, in modern Japanese it is pronounced basically the same as お.

支払期限 is a compound noun made of:

• 支払 = payment
• 期限 = deadline / time limit

So together it means payment deadline or payment due date.

You can think of it as one noun phrase:
支払期限 = the payment deadline

を marks the direct object of the verb.

Here, the verb is 確認する (to check / confirm), and the thing being checked is 支払期限.

So:

• 支払期限 = what is being checked
• を = marks it as the object
• 確認する = check / confirm

In other words, this follows the common pattern:

noun + を + verb

確認する is the plain/dictionary form.

This form is often used in:

• headings
• instructions
• manuals
• to-do lists
• simple written statements
• dictionary examples

So 支払期限を確認する。 can feel like:

• a neutral statement
• a checklist item
• an instruction such as Check the payment deadline

If you wanted polite spoken Japanese, you might say:

支払期限を確認します。

Yes, the subject is omitted, which is very common in Japanese.

Japanese often leaves out:

• I
• you
• we
• someone

when it is obvious from context or not important.

So depending on context, this could mean:

• I check the payment deadline
• You should check the payment deadline
• Check the payment deadline

The exact subject has to be understood from the situation.

Normally, Japanese is written without spaces, so this would usually appear as:

支払期限を確認する。

The spaced version is often used only for learners, formatting, or clarity. Native Japanese writing generally does not separate words with spaces.

確認する is a suru-verb.

That means:

• 確認 is a noun meaning confirmation / checking
• adding する turns it into a verb: to confirm / to check

This is a very common pattern in Japanese:

• 勉強する = to study
• 連絡する = to contact
• 確認する = to confirm

So 確認する is literally something like to do confirmation, but in natural English it is simply to check / confirm.

Not exactly.

確認する usually means:

• to verify
• to make sure
• to confirm something is correct

It sounds more deliberate and formal than 見る.

Compare:

• 見る = to look / see
• 確認する = to check carefully / confirm / verify

So if you are checking a deadline to make sure you have the correct date, 確認する is a very natural choice.

They are very close in meaning, and both can be used.

• 支払期限
• 支払い期限

Both mean payment deadline / due date.

However, 支払い期限 may feel a little more explicitly noun-like to learners because 支払い is clearly the noun form of 支払う. In real Japanese, both are seen, especially in formal or business contexts.

If you are learning, it is useful to recognize both.

Yes. In isolated written Japanese, the plain form can sound like an instruction or reminder.

For example, on a checklist or in a manual:

• 支払期限を確認する。 = Check the payment deadline.

In conversation, though, if you are directly telling someone to do it, you would more often use forms like:

• 支払期限を確認してください。 = Please check the payment deadline.
• 支払期限を確認して。 = Check the payment deadline. / Please check it. (casual)

This happens because 支払 here is being used as a noun-like stem inside a compound.

The full verb is:

• 支払う = to pay

But in compounds, Japanese often uses the stem form:

• 支払
  • 期限 → 支払期限

This is very common in Japanese vocabulary. A verb-related idea often appears in compounds without its dictionary-form ending.

By itself, it sounds a little written or note-like rather than conversational.

In everyday speech, people might say things like:

• 支払期限を確認する。 — neutral, written, or self-directed
• 支払期限を確認します。 — polite statement
• 支払期限を確認してください。 — polite request
• 支払期限、確認した？ — casual conversational style: Did you check the payment deadline?

So the original is natural, but its most natural setting is often written language, instructions, or concise statements.