You may notice a difference between your spoken and your written language. In speech, people tend to use and and so as all-purpose connectors.
He enjoys psychology, and he has to study hard.
The meaning of this sentence may be perfectly clear in speech, which provides clues with voice, facial expressions, and gestures. But in writing, the sentence could have more than one meaning.
Although he enjoys psychology, he has to study hard.
He enjoys psychology although he has to study hard.
The first sentence links two ideas with a coordinating conjunction, and; the other two sentences link ideas with a subordinating conjunction, although. A coordinating conjunction gives the ideas equal emphasis, and a subordinating conjunction emphasizes one idea more than another.
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Exercise > Coordination and subordination
LearningCurve > Coordination and subordination