In English it is pretty easy to make sure your verbs and subjects agree. Basically you check to ensure that singular subjects have singular verbs and plural subjects have plural verbs. But what kind of verb should you use in sentences that have two subjects?

Sentences with two subjects joined by and take a plural verb, even if each of the two subjects is singular. (Think of math: one + one = two. One subject + one subject = plural subject.)

Here are some sample sentences with subjects joined by the word and:

The sofa and the pillow are very comfortable. (sofa + pillow = plural subject, are = plural verb)
The picture and its frame belong together. (picture + frame = plural subject, belong = plural verb)
Romance and garlic do not mix. (romance + garlic = plural subject, do mix = plural verb)

Which sentence is correct?

A. The judge and the jury have shown no mercy in these cases.
B. The judge and the jury has shown no mercy in these cases.

Answer: Sentence A is correct. The subject is plural (judge and jury) so a plural verb (have shown) is appropriate. In sentence B, the verb (has shown) is singular.