Which sentence is correct?
I observed my colleagues operate this instrument.
I observed my colleagues operating this instrument.
Both are correct. If you use the -ing form, you are focusing on observing the on-going operation of the instrument as it was happening. It's a more detailed view of what you observed.
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Both are correct.
If you use the -ing form, you are focusing on observing the on-going operation of the instrument as it was happening. It's a more detailed view of what you observed.
If you use the plain form, you are focusing on the operation of the instrument as an event (or separate individual events) that you observed from beginning to end. It's a less detailed view of what