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 notuse 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 if is pe which has been reconstructed. The word following pe does not mutate.
Examples: Pe nin melog, darthathog = If you love me, you will stay Pe han iestog = If you wish that ú-'watham pe eliatha = We're not going if it rains
You may also see the word ae = if used. This was reconstructed by David Salo for the films.
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 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 iaul losta ias aníra = The cat sleeps where it wants