Hi Victoria, This is a subtle thing in English, but I think I can explain it to you. You are correct that we often use "in" for parts of the day. Here are two examples: 1) Can I call you tomorrow in the morning? (this is a specific morning - tomorrow morning) 2) I like to drink coffee in the morning. (here we're talking about mornings, generally) So "in the morning" can refer either to a specific morning or to all mornings, but notice that both these examples use "in the." Now here are two more examples: 1) I hate getting out of bed on a rainy morning. 2) It happened on a morning just like this. Here you are talking about a specific kind of morning. Notice that there are other words to describe the morning ("rainy," "just like this") and also notice that you use "on a." So... "in the morning" can mean one specific morning or all mornings. You can say "it happened in the morning" and it makes perfect sense. "On a morning" needs more information. What kind of morning? You would never say "it happened on a morning." You need more information to understand what kind of morning. "It happened on a Tuesday morning," "it happened on a winter morning," "it happened on a foggy morning," etc. Remember, "in + the + morning" but "on + a + morning + more information about what kind of morning you're talking about." I hope that helps. Let me know if you have more questions.

Hello Victoria. I will try to explain it for you. The winter evening and a sunny morning are moments/events here. Without a time period! Do not think of time, just a moment or event! Your examples that you have put down, that use the preposition IN have time periods 'in the morning/afternoon or in the evening' are said with a time period in mind. Example, "i will call you in the afternoon" When will i call you? During the time period between 12pm and 6pm. A TIME PERIOD of the afternoon! So, your sentence - happiness is sitting on a sofa on a winter evening with a box of chocolates, means that the happiness is sitting on a sofa at this particular event that is called a winter evening. Another way., you could think about it is literally doing the action of being ON something. So, imagine that 'the winter evening' is a carpet laying on the ground in your living room. You get up on top of a chair and you JUMP on top of the carpet. You have now landed ON the carpet! So, you have landed on top of the moment, the event, 'the winter evening'. I hope this helps

Hi Tran, You are right to use In with the parts of the day. In English we use ON to talk about parts of specific day of a week , a month or a season as you mentioned above ,for example ON MONDAY MORNING, ON A SUNDAY or ON A COOL SEPTEMBER EVENING....... Best regards.