Help me about using oscilloscope ?

[Nhan95]

When I use analog oscilloscope in the experiment, sometimes I work with sine wave or square wave, my teacher asked me to choose DC mode for channel 0 and channel 1 or I will face with wrong result. Uhm, I feel confused .  Could you help me when we use DC or AC mode?

[xrunner]

If the sine wave or square wave has a DC bias either pos. or Neg, then you need to be DC coupled or you won't be able to see the offset. AC coupling will remove any DC biasing of a waveform.

[Paul Moir]

Another way understanding it is to see what the switch actually does.  When I was younger, I could only afford old tube based 5MHz bandwidth oscilloscopes, which were pretty horrible but how they worked was obvious just by looking.  The AC coupling switch simply put a 0.1uF capacitor in series with your probe lead.  That way all DC was blocked to the vertical amplifier.

[Lightages]

Before asking for any help with basic use of an oscilloscope, everyone should watch the video in this thread.

https://www.eevblog.com/forum/beginners/oscilloscope-training-class-(long)/

[pascal_sweden]

Or you could download this document from Tektronix
http://info.tek.com/www-xyzs-of-oscilloscopes-primer.html

[xrunner]

...my teacher asked me to choose DC mode for channel 0 and channel 1 or I will face with wrong result.

I'm thinking - is the teacher teaching you these things? Did you ask the teacher? Just thinking out loud on this end.