What is the difference between in work and at work? Does in work imply that the person is busy and immersed in his work?
I am at work.
He is in work.
"In work" means I have a job. I might not be there currently, but I have employment. "At work" means I am currently doing my job, or at least on the premises of my job.
I upvoted a comment yesterday that I believe is worthy of repeating:
Indeed, in work can mean exactly what you said:
But in work can be used in many other contexts as well:
At work is similar, in that there are several possible nuances:
1Interestingly enough, this one could mean "He stayed working until 7:00" or "He stayed at his place of employment until 7:00."
An American would be very unlikely to say, "He's in work."
"He is at work" usually means he is presently at his place of employment. It can also mean he is presently engaged in work. Like if someone is at home but doing something related to his job, and an interruption comes along, he might say, "I'm at work -- tell them to call back later".
I can't think of a general rule or pattern for when you would use "in work". It is almost always used with a verb like "engaged" and some sort of descriptive phrase to describe the work. Like, "He is engaged in dangerous work", or "He is tied up in work that is very tedious and boring."
from the Longman Language Activator:
be in work [British]: to have a job - use this when you are comparing someone who has a job with other people who do not have jobs
She was the only one in the family to be in work.
be at work: to be doing your job at the place you work, especially at a factory, office etc owned by your employer
What time do you have to be at work?
As you can see, "to be in work" is not a common idiom in American English.
The idiom "in work" is quite common in the aerospace industry, and is often heard in space-to-ground and space-to-space radio transmissions, e.g. "Step 2 on the post-docking checklist is complete, and step 3 is in work." Astronauts with a task IN work are usually hard AT work.