What's the difference between "chief" and "captain"?

Could you teach me when can I use “no longer”?

The expression "no longer" is used when something which was once true or possible isn't true or possible anymore. Ex: The restaurant is no longer open. Ex: I just found out I am allergic to dairy, so I can no longer drink milk.

What's "premiere"? and what does prefix "pre-" mean?

A "premiere" refers to the very first showing of a movie, play, or performance, usually only for the cast, crew members, and special guests. The prefix "pre-" means before or previous to something.

Do you just say "morning" not "good morning"? Is it acceptable?

Just saying "morning" is more casual than "good morning". You'd be more likely to say 'morning" to people like friends or strangers than to professors or colleagues. "Good morning" is still the standard, but both are acceptable. Ex: Morning guys. I'm so glad we could hang out today. Ex: Good morning class.

What's the meaning of "defiance"?

"Defiance" means a daring or bold resistance to any opposing force. In this case, the penguin chick is bravely opposing the Petrel. Ex: The nurses went on strike as an act of defiance. Ex: He disobeys his parents as an act of defiance.

What does "step up" mean here?

ここでの「step up」とは、助けが必要なときに行動を起こす、率先して行動することを意味します。 「step up」とは、何かに一生懸命取り組み始めることも意味します。 例：He stepped it up at work to try and get the promotion.（彼は昇進できるように、仕事への取り組みを強化しました。） 例：After their father died, the oldest son really stepped up and took on more responsibilities to take care of the family.（父親が亡くなった後、一番上の息子が懸命に努力し、家族の世話をするためより多くの責任を引き受けました。）