Bill and his horse were riding in the desert.

[lagoo]

I can understand that if the sentence was "Bill was riding his horse in the desert".

But I came across this sentence "Bill and his horse were riding in the desert" on a TV program which has confused me.

Did that still mean that Bill was riding the horse?

[Rover_KE]

We can assume that.

It's unlikely to be the other way round.

[andrewg927]

Not a teacher.

Yes. It means Bill was riding the horse.

[jutfrank]

It could mean that they were both on bikes. But without more context we assume this is not the case, of course.

Still, this unintended ambiguity makes it a badly-written sentence, in my judgement.

[Rover_KE]

andrewg927, please read this extract from the posting Guidelines:

You are welcome to answer questions posted in the Ask a Teacher forum as long as your suggestions, help, and advice reflect a good understanding of the English language. If you are not a teacher, you will need to state that clearly in your post. Please note, all posts are moderated by our in-house language experts, so make sure your suggestions, help, and advice provide the kind of information an international language teacher would offer. If not, and your posts do not contribute to the topic in a positive way, they will be subject to deletion.