Learn Swedish: Jag dricker gärna kaffe på morgonen. - I gladly drink coffee in the morning.

In English, we usually say I like to drink coffee, using a verb. In Swedish, it is very common to use the adverb gärna (which translates to gladly or willingly) right after the action verb instead. So Jag dricker gärna is the most natural way to say I like to drink or I like drinking.

Why is morgonen in the definite form? Doesn't this mean one specific morning?

When you combine på with a time of day in its definite form (like morgonen), it describes a habit or something that happens repeatedly. So på morgonen means in the morning or in the mornings in general, rather than just one specific morning.

Could I put gärna at the end of the sentence instead?

Usually not. Gärna is a sentence adverb, and in a standard main clause, it belongs right after the verb. Putting it right after dricker is the correct word order here: Jag dricker gärna kaffe.

How is på morgonen different from i morse?

På morgonen refers to mornings in general as a daily routine. I morse specifically means this morning and is always used to talk about the past. If you wanted to say I drank coffee this morning, you would change the verb to the past tense and say Jag drack kaffe i morse.