This road is in very bad condition. OR: This road is in a very bad state. But NOT: This road is in a very bad condition.

In the sentence "The road is in very bad condition." the phrase "bad condition" indicates a non-discrete property. It is similar to "less water". It is not proper to say "fewer water"; you should instead say "less water". For discrete entities like items or states, you can refer to "fewer items" or "a state".

Both alternatives are perfectly OK, and mostly mean the same as each other. Arguably 'good condition' is somehow longer-term, whereas 'a good condition' is implicitly one condition of many, hence more transient. For example, if I am in bad condition, that probably means I get little exercise, and eat and drink too much: this is a semi-permanent state. If I am in a bad condition, that might mean I broke my arm this morning. This distinction is not always relevant or apparent - if i were describing the condition of a car or road I would use them interchangeably.

This sentence is perfect --- "This road is in very bad condition." Hope this helps!

Thank you, Sergio! could you explain why?