Hi Anil
Yoong Liat has:
“I’m very sorry. I can’t stay just now, for I must be back at school by two o’clock. May I call again later in the afternoon?”
(To me, "I'm very sorry" is by itself a complete sentence, so a full stop is required after 'sorry'. Do you notice there is a comma after 'now' followed by the next word 'for'? This is because when 'for' is used with its meaning 'because', a comma needs to be inserted before' for'. ( For example, The baby is crying, for she is hungry.)
Clive has:
"I’m very sorry, I can’t stay just now for I must be back at school by two o’clock. May I call again later in the afternoon?”