I want to pause an activity. How can I do this?

you can use the suspend/resume dates fields. The date work has temporarily stopped on a given activity.

For example, an event might force an activity to be interrupted or resource constraints might put an activity on hold. The suspend date must be later than the actual start date, which the activity must have. Activity progress can resume when the Resume Date is set. The suspend/resume period behaves like non-work time on the activity calendar or resource calendar for task and resource dependent activities.

Resume Date field -The date when a suspended task or activity is resumed.
The resume date must be later than the suspend date and earlier than the actual finish date.The suspend/resume period behaves like non-work time on the activity calendar.