1. Helpdesk: place in a company where support and assistance is offered to its users. (Is this wrong?)
2. Helpdesk: place in a company where support and assistanceare offered to its users. (If #1 and #2 are correct, how does #2 differe from #1 and how do I know #1 is not wrong?)
3. Helpdesk: place in a company where support or assistance is offered to its users. (With 'or' here do I use 'is' or 'are' ? Why?)
4. Helpdesk: place in a company where support or assistance are offered to its users. (I have 'support' and 'assistance' here but 'are' sounds odd in this sentence? Why? How does the subject and verb work here?)
5. Helpdesk: place in a company where supportand/orassiatnce is offered to its users. (Do I use 'is' or 'are' here? How do I know?)
One more thing:
Are these correct? If not, why?
6. How do the subjectandverb work here? (Is this correct? It sounds a bit odd?)
7. How does the subject and verb work here? (Is this okay colloquially? In other words, is it 'How does the subject and how does the verb work here?' ?)
8. What are the subject and verb here? (Is this correct? It sounds a bit odd? If so, why? It sounds like it should be 'What are the subjects and verbs here?' If so, how come #6 is okay?)
You can use both the singular and the plural in 1 & 2. It depends on whether you see support and assistance as two separate things or as a single thing. With 'or' the plural sounds a bit strange to me because it suggests that only one thing is offered at a time. With number 5, I'd say you could use either.
With 6 & 7, again, I'd say either could be used. 8 is fine because you have two things, but, again, many would use a singular verb.
I should point out here that I am a British English speaker, and we tend not to care very much about singular and plural. Americans may well have a different view.