Relative pronouns are the words 'that/who/whom/which'. The relative pronoun in Sindarin is i for singular and in for plural, exactly like the word 'the', except you do not use soft mutation on the following word (or nasal mutation if it's plural). The relative pronouns are marked in red.
Attested examples: i sennui Panthael estathar aen = who should be called Fullwise ae Adar nín i vi Menel = my (our) father who is in heaven
More examples : Cenin i tellinil na Imladris = I see that you came to Imladris Den cenithon i melon = I will see him whom I love Se i mar vi i lostam = This is the house in which we sleep
What stops this i from meaning 'the'? If you translate the first sentence back using i as 'the' you would get 'I see the you came to Imladris', which doesn't make sense! As always, context is everything.
How do I know when to use i for who and when to use a man? This is a good question, and easily answered. A man is an interrogative pronoun, used for asking questions. Here, we are not asking anyone a question, and so we use the relative pronoun, i.
The word for when/while is ir. The word following ir does not mutate.
Examples: Istathon ir telig na Lothlorien = I will know when you come to Lothlorien Farathon ir postol = I will hunt while you rest
The word for where is the reconstruction ias. The word following ias does not mutate. Remember this is not for questions like "Where is the dog?", only sentences of the type below!
Examples: Iston ias darthog = I know where you stay I vŷg losta ias aníra = The cat sleeps where it wants
We have two methods of expressing uncertainty in Sindarin, and both of them are reconstructed.
The word for if is pi. The word following pi does not mutate.
Examples: Pi nin melog, darthathog = If you love me, you will stay Pi han iestog = If you wish that ú-venatham pi eliatha = We cannot go if it rains
You may also see the versions ae - 'if' and pe - 'if'. Ae was reconstructed by David Salo for the films and Pe is an older version of Pi.
Might, maybe, perhaps
The word for this is ce. The word following ce does not mutate.
Examples: Ce eliatha abor = Perhaps it will rain tomorrow I orn ce brûn = The tree might be old I vess ce maethor = The woman might be a warrior