since +一段时间+ ago,前面主语难道必须要用现在完成时态吗

for和since是现在完成时的提示词汇，但两者用法不同：
(1)for和表示一段时间的词组连用，如：
for three minutes, for six hours, for nine days
for twelve weeks, for fifteen months, for thirty years
(2)since和表示过去某一时刻的词或词组连用，如：
since nine o'clock this morning, since three weeks ago
since last Friday, since September

Since i have been busy some time ago,therefore i have not have a chance to call you

必须，因为Since 引导完成时

必须