"I thought we could use "the" the second time we talk about the same thing we just mentioned." No. You thought wrong. That's sometimes the case, but it's not what definite articles are for. In fact, we use 'a' to mean '(any) one (of several)'; we use 'the' to refer a SPECIFIC one. 'The' is a definite article, used when we're definite about which person or thing we're referring to. We would only say "the road sweeper earns a great deal of money" if there were only one roadsweeper in the city (or country, or whatever the context is). "The road sweeper" would refer to the one and only person who did that job. Compare with this: "He was offering me the job of Chairman. He said that the Chairman earns a great deal of money." Here we talk about 'the job of Chairman' and 'the Chairman earns' because there is only one Chairman in this context - we know that we are referring to only one specific person/role. Obviously, there isn't only one road sweeper around. This is why we say "a road sweeper earns a great deal of money" - it means any one of the many people who are employed to do this job. Another possibility is the plural "..road sweepers earn a great deal of money". This means road sweepers in general. Does that make sense? If it does (or even if it doesn't) , do please acknowledge this response, Mr Tiger. Asking people to help you and then not responding to the answer is rude.