If you know someone who wishes to change the pronoun by which they are referred (for example from she to he), try to understand it is very important to them that you respect this, even if you initially find it difficult to consider them that gender. Deliberately using a different pronoun for the person will be experienced as offensive, judgemental and hurtful.
Most trans people will use the most common pronouns, ‘he’ and ‘she’, to refer to themselves. However, some people may use the gender neutral pronouns ‘they’ and ‘their’ in the singular sense. You may also meet a trans person who uses less common gender neutral pronouns (such as ‘zie’ or ‘hir’) but these are currently mostly used online. In place of the gendered titles of address (Mr, Miss, Mrs, Ms), you might see the use of a newly created gender-neutral title of Mx. More commonly people may just opt not to use any title. Once someone has let you know their pronouns and title, it is really important to try and get them right as much as possible, even if they are new or unusual to you.
If you are speaking briefly with someone and you are unsure how the person would wish to be addressed, then it is usually best just to avoid using any gendered terms than to risk insulting them by guessing wrong.
When the interaction is long enough, ask the person their name to try to determine which pronoun to use. If it is still not clear then it is acceptable to politely ask: “excuse me, but which pronouns do you use?” or “excuse me, but how do you like to be addressed?”
If you accidentally use the wrong pronoun, just apologise once and continue with the conversation. You don’t need to apologise profusely or try to explain why it happened – this often only causes more awkwardness.
When referring to a transgender person in their absence, you should still ensure you use the correct pronouns. This is important in order to be respectful of their identity and also maintaining the consistency will help to prevent confusion, uncertainty and embarrassment for everyone.
Occasionally the situation can be a little more complex: A transgender person may prefer one pronoun and name in some situations, and a different name and pronoun in others. In such cases it is still very important to try and get it right. If you are ever unsure, ask them again – don’t just guess. It is also important not to link the names together as that could lead to them being outed.