One of the biggest differences between English and Spanish revolves around how verbs change (or don’t change) when conjugated for different tenses and persons.
In English, verbs change almost exclusively according to their tense, while in Spanish they change according to both their tense and the person for which it is conjugated. The closest equivalent in English would probably be the way verbs change in present tense for the 3rd person singular by getting an extra ‘s’ at the end, as in “he eats”, “she runs”, “it works”.
The change in Spanish is more dramatic. Let’s pick as an example the verb “comer” (to eat) and see how it is conjugated in present tense:
“yo como” “nosotros comemos”
“tu comes”* “ustedes comen”*
“él/ella come” “ellos comen”
*There are some regional variations here, I’m using “standard” latinamerican Spanish for this example.
As you can see, the verb has a different termination that matches the person for which it is conjugated (with the exception of the last two persons of plural, which have the same ending). This makes it possible to use a verb without including its subject, since it can be inferred by context. This is called “sujeto implícito” (implicit subject).
“Juan tiene mucha suerte. Come muy bien, pero no engorda”
(Juan is very lucky. He eats well, but doesn’t get fat)
As you can see, only the first phrase includes the subject explicitly, “Juan”. The second phrase doesn’t have a subject, but it can be inferred from the first phrase, and from the fact that the verb (“come”) is conjugated in a manner that is congruent with that.
Evidently, there are cases where using an implied subject can introduce ambiguity (which may or may not be intentional). If you want to read more about that, check out this post.