Not all the rules of Spanish are straightforward or logical, and when it comes to the use of number-verb agreement with gustar, the rules aren't always followed, as the following Q&A makes clear.

Question: I have a question about gustar. I have looked for an answer in books and online, but I either can't find an answer or get conflicting ones. In a sentence with two singular subjects, such as "Me gusta la hamburguesa y el queso," would it be correct to use the singular form of gustar or the plural (gustan)?

Answer: You could defend either choice in a sentence like that. Using gustan would certainly seem logical, and it is indeed said that way sometimes. But it is far more common to use the singular, gusta. It's kind of like shortening "me gusta la hamburguesa y me gusta el queso" by leaving out the second "me gusta," just as in English we might shorten "the happy children and happy adults" to "the happy children and adults." Why say "me gusta" twice if once gets the message across?

According to the Royal Spanish Academy, the singular verb should be used in a sentence like this when the two things you're talking about are uncountable or abstract and they follow the verb (as is usually the case with gustar). Here's an example the Academy gives: Me gusta el mambo y el merengue. Note how the two subjects are uncountable (they are both types of music or dance). Here are some other sentences that follow this pattern:

• Es una red social de gente que le gusta el deporte y el ejercicio. It's a social network of people who like sports and exercise. 

• Me encanta el manga y el anime. I love manga and anime.
• Me gusta la música y bailar. I like music and dancing.
• Al presidente le falta el coraje y la voluntad política para resolver los problemas de nuestro país. The president lacks the courage and political will to resolve the problems of our country.
• Si te gusta el cine y la tele, querrás pasar tiempo en California. If you like movies and TV, you will want to spend time in California.

But the Academy would pluralize the verb if the objects are countable. One of the Academy's examples: En el patio crecían un magnolio y una azalea. A magnolia and an azalea grew in the courtyard..

Other examples of the Academy's preference:

• A ella le encantan la casa y el parque. She loves the house and the park.
• Nos bastan el ratón y el teclado. The mouse and the keyboard were enough for us.
• Me gustan ese camisa y ese bolso. I like that shirt and that purse.

In real life, however, the singular verb (when it precedes two subjects) is used much more often than the Academy would suggest. In everyday speech, even when verbs such as gustar have two countable subjects, the singular verb is usually used. In the following examples, both sentences might be said by native speakers, but the first is more commonly heard even though the second one is grammatically preferable to the Academy:

• Me duele la cabeza y el estómago. Me duelen la cabeza y el estómago. I have a headache and stomachache.
• Me gusta mi cama y mi almohada. Me gustan mi cama y mi almohada. I like my bed and my pillow.
• A Raúl le gustaba el taco y el helado. A Raúl le gustaban el taco y el helado. Raúl liked the taco and the ice cream.

As to your example, if by hamburguesa you mean ground beef, both of the subjects would be uncountable and the Academy would prefer you use the singular verb, gusta. If you're referring to a type of sandwich, which is countable, the Academy would prefer you use the plural, gustan. In real life, however, you're not going to get any flak from me if you use either one.