Relative Pronouns

Replace "who", "whom", and "that" with كه /keh/

1. The man who is walking is my friend.
man = مرد /mard/
who = كه /keh/
to walk = قدم زدن /ghadam zadan/
is walking = دارد قدم مي زند /da:rad ghadam mi zanad/
my friend = دوست من /dust e man/

The only change we need is that we should put /i:/ sound or simply the letter ي at the end of the 'noun' (/mard/). So, instead of saying /mard keh/, we should say مردي كه /mardi keh/. This rule is applicable to all kind of relative pronouns.

Now, the translation:
مردي كه دارد قدم مي زند دوست من است

2. The man who is sitting in the chair is my father.
is sitting = نشسته است /neshasteh ast/(note: here we use 'Present Perfect Tense')
in = on = روي /ru:ye/
chair = صندلي /sandali/
my father = پدرم /padaram/ = پدرمن
مردي كه روي صندلي نشسته است دوست من است /mardi keh ru:ye sandali neshasteh ast pedar e man ast/

3. The man whom you saw is my friend.
saw = ديدي /didi/ (simple past tense)
مردي را كه تو ديدي دوست من است
Note: we'd better put را /ra:/before كه/keh/ whenever we are to use 'whom'.

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