1.They happily go to Beijing to attend a meeting. 2.They go to Beijing to attend a meeting happily. These two sentences don't make sense. You are using the present simple wrongly here. 3.They went there happily 4.They happily went there These two are fine.

hi, 1. They are happy to go to beijing but 2. they are happy to attend the meeting all the other examples you have given mean the same. the first one is different because, it has two main verbs. They go to Beijing and attend the meeting. Use adverb i.e. happily for the verb which is making them happy the rule is rather simple as above. The adverb must be near the verb which it is describing. because as you see in the name of "adverb" it should be added to the verb. In case of sentences with one verb, the positioning doesnt matter but be careful and place the adverb just infront or after the verb sentences with multiple verbs good luck :)

They sound correct. No one would think this sounds strange even if they want it written differently.