Reminder vs memento
Which word is better to use when I talk about something that reminds me about days passed with someone. For example: "This photo is a nice reminder / memento of those summer days." For me it feels like a reminder is like a note, that you make to remember something, whereas a memento is more like a souvenir that you bring, say, from travel. Though, in this case a photo is neiter a note nor a souvenir ...

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You could use either of the two words here. If you use "memento", I would use the preposition "from" (This photo is a nice memento from those summer days"). I would venture to say that a memento is something that will always be cherished and typically has a positive connotation to it, while "reminder" can be both positive and negative.

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I don’t think of pictures as mementos.  I think of objects and souvenirs as mementos.  I would say reminder or memory.