Breakdown of Он долго не мог успокоиться.
не
not
он
he / it (for a masculine noun) / it (for a neuter noun)
долго
for a long time
мочь
to be able to
Questions & Answers about Он долго не мог успокоиться.
Why does успокоиться end in -ся, and how does it fit with verbs of emotion?
The -ся ending makes the verb reflexive. Similar to verbs in this lesson like радоваться (to be glad) or удивляться (to be surprised), успокоиться describes an internal emotional state happening to the subject. Without the -ся, the verb успокоить means to calm someone else down.
Since долго (for a long time) usually triggers imperfective verbs, why is успокоиться perfective?
The word долго is actually modifying the imperfective verb мог, not успокоиться. He spent a long time being unable (не мог) to achieve the final, successful result of getting calm (the perfective успокоиться).
Could we use the perfective не смог instead of не мог here?
Not in this exact sentence. Не смог indicates a single, definitive failure to do something. Because the word долго shows that his struggle lasted over a continuous period, you must use the imperfective не мог to show an ongoing inability.
What is the imperfective pair for успокоиться?
The imperfective form is успокаиваться. You would use it to talk about the ongoing process rather than the final result. For example, он медленно успокаивался means he was slowly calming down.