What Am I Riddles

Question: Take my first and make it my last, now I happened in the past.

What am I?

Question: I am a teacher, yet I am not a human nor a robot.

Question: I am weightless, but you can see me. Put me in a bucket, and I'll make it lighter.

What am I?

Question: I plow and plow, but never sow.

What am I?

Question: I used to fill rooms but now I'm a tower,
I can stay put or I can travel far.

What am I?