Present tense

In general, use present tense rather than future tense; try to avoid using "will" where possible. For example:

Not recommended: Send a query to the service. The server will send an acknowledgment.

Recommended: Send a query to the service. The server sends an acknowledgment.

Sometimes, of course, future tense is unavoidable because you're actually talking about the future. (As in "The hedgehog will hibernate starting in October.") Much of the time attempting to predict the future in a document is a bad idea, but sometimes it's necessary.

But the fact that the reader will be writing and running code in the future isn't a good reason to use future tense. Stick with present tense where possible, as shown in the "Recommended" example above.

Also avoid the hypothetical future "would." For example:

Not recommended: You can send an unsubscribe message. The server would then remove you from the mailing list.

Recommended: If you send an unsubscribe message, the server removes you from the mailing list.

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Google Developer Documentation Style Guide
Google Developer Documentation Style Guide