Generally, we miss friends, or buses, or targets. * I miss my friends because they’re so far away. * I missed the bus because I left the house too late. * I’ve missed my targets every month this year. I’m worried I might lose my job. However, we miss out on things like opportunities. * She missed out on the opportunity to study overseas because she submitted her application too late. If you “miss out”, you fail to use an opportunity or get an advantage from something: * We didn't have a TV at home when I was young, and I felt as though I missed out. https://dictionary.cambridge.org/dictionary/english/miss-out

Hi Allan Chen As Gareth explained we use 'miss' and 'miss out' in different ways. This is because 'miss out' is a phrasal verb. You MUST always think of phrasal verbs as having one unit of meaning, which is not the same meaning as the words 'miss' and 'out' . In this way phrasal verbs are a bit like compound nouns. For example the words 'play' and 'station' do not have the same meaning as 'Playstation'. Although 'miss out' is two words it actually has one meaning, and that meaning is different to the word 'miss'. As you noted we normally say 'miss out' + 'on' something. Because 'to miss out' means you never had the opportunity to do or experience something. 'Miss' means to fail to hit/catch/reach/be present Take your last example. If someone said "I missed out on the train" you could express the same idea by saying "I never had the opportunity to catch the train" - the two sentences mean the same thing. BUT this would be quite a strange thing to say. After all, you did have the OPPORTUNITY to catch the train but you did not catch it because you were late - therefore the correct sentence is "I missed the train." I hope this helps. Chris