And yes, it's harder if you knew the person pre-transition. And it's also harder if they're earlier in their transition and their presentation resonates in your brain as different from their preferred pronouns. It's harder for them too.
And it may be hard when telling a story and they went by a different name/pronouns in that time. It may be confusing.
But it's really not a big ask to call someone by the name/pronouns that they want.
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