You must be able to prove one of the following:

you’re in a civil partnership or marriage that’s recognised in the UK
you’ve been living together in a relationship for at least 2 years when you apply
you are a fiancé, fiancée or proposed civil partner and will marry or enter into a civil partnership in the UK within 6 months of arriving
you’ve been in a relationship for at least 2 years when you apply but you cannot live together, for example because you’re working or studying in different places, or it’s not accepted in your culture
You also need to prove you:

have a good knowledge of English
meet the financial requirements
If you cannot provide this proof you may still be able to apply for a visa or extend your permission to stay if:

you have a child in the UK who is a British or Irish citizen or has lived in the UK for 7 years and it would be unreasonable for them to leave the UK
it would breach your human rights to stop you coming to the UK or make you leave
If you’re applying as a fiancé, fiancée or proposed civil partner

You must prove that:

any previous marriages or civil partnerships have ended
you plan to marry or become civil partners within 6 months of arriving in the UK
You will not be able to work or study during your engagement.

You’ll need to apply for permission to extend your stay in the UK when you marry or enter into a civil partnership. If your application is approved, you’ll have the right to work or study in the UK.