In this sentence, the speaker is issuing Ian a command:

"Ian, set course for Mars Station."

This violates the rule about separating a noun from its verb with a comma. However, written:

"Ian set course for Mars Station."

it does not sound like a command to my ear, but like a statement of fact.

Is it incorrect to use a comma in this instance? If so, how can I prevent the sentence from reading like a statement rather than a command?

  • It is correct to use a comma after "Ian". I don't know what rule "about separationg a noun from its verb" you're talking about. – CowperKettle Dec 18 '16 at 7:10
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    The rule you're thinking of states that a subject should not be separated from its verb. The addition of a comma means that" Ian" is not the subject (think of the "rule") but a vocative, an optional item used merely as a means of address, and "set course for Mars Station" is then an imperative with its own subject (unstated but understood as "you"). So, if you want the clause to be an imperative, then keep the comma after "Ian". But if you want it be a declarative past tense clause, then omit the comma making “Ian” the subject. – BillJ Dec 18 '16 at 9:53
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    Indeed, that comma is actually called a vocative comma. It's very necessary. – Andrew Leach Dec 18 '16 at 10:30

Keep the comma, because removing it actually makes it a statement.

Ian set course for Mars Station.

The above notifies the reader of the past actions of Ian.

Ian, set course for Mars Station.

Here, the speaker is telling Ian to set course for MS. Imperative sentences often appear to be missing subjects and use a verb to begin the sentence. In fact, the subject is the person listening, or the audience. Here, Ian is the person listening and rest of the part is the command given to him, that is, an actual imperative sentence. The subject-verb-seperation rule needs not be applied in every case.

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    In the second example, "Ian" is called a vocative, an optional item used as a means of address. But "Ian" is not the subject, or even part of it; the imperative "set course for Mars Station contains its own subject which is unstated, but understood as "you", i.e. "(You) set course for Mars". – BillJ Dec 18 '16 at 10:27

You should note that in an imperative sentence, the subject you is elided because it is not necessary unless you want to emphasize it. For example,

Go there.

You go there.

If you don't place a comma, Ian will be a subject of a declarative sentence as follows:

Ian set course for Mars Station.

Also, you need to note that if you don't place a comma, the sentence is in the past tense as the verb set doesn't use the third-party singular form sets as in:

Ian sets course for Mars Station.

In the imperative, a person's name is an interjection and it is recommended to place a comma after or before an interjection.

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