The trim function in AutoCAD is very useful but it is commonly not understood or used correctly. Here are some helpful tips on using TRIM.

<command> TRIM or TR <enter> on the RIBBON – Home tab/ribbon > Modify panel > Trim button

If you do the above command or push the button, you have two options 1) “Select objects” or 2) hit enter one more time to “<select all>.”
1) SELECT OBJECTS

• Think of this as selecting the edge that you want to trim to or trim between.
• Pick the line or lines that will be the cutting edges and hit <enter>.
• Now select the line or lines that cross the cutting edge and hit <enter>
• Note – the side of the edge you select will be trimmed.

In the picture below, #s 1 & 2 are the cutting edges and #3 is the object that I want to trim.

2) <SELECT ALL>
This is also known as “TRIM <enter> <enter>”
What this does is instead of individually selecting your cutting edges, everything in the drawing is selected as a cutting edge.
In the above picture, if the TRIM <enter> <enter> function was to be used to create the same results , you would have to select each portion of the circle between the lines.
There area times when TRIM <enter> <enter> is useful, but knowing when to use both functions will save you time.

Drafters use the trim function so much that they have added an EXTEND function into TRIM. To do this:
TRIM <enter> <enter> then hold down the SHIFT button, then pick the side of any line(s) and the line will extend itself to the nearest object that is in the line’s path. To make the line go further, just keep clicking on it until it extends to the desired object.

TRIM with a “Fence”
This trim function may sound confusing, but it is really just drawing a polyline through other lines and objects, and the segment where the “fence” (polyline) crosses, it trims.
See the picture below. the greens line is the “fence” and it passes through the overhanging lines and trims them.

BONUS!!!    EXTRIM
The command EXTRIM is not found in the ribbon. It is just one of those secret commands that people wish they knew yesterday.
It is great for trimming little lines that  are too small to see or large amounts of overhanging lines.
To use EXTRIM:

• <command> EXTRIM <enter>
• Select the boundary edge
• Pick the side of the boundary that you want to be trimmed. Everything on the side that choose that crosses the boundary will be trimmed.

See Picture Below:  The yellow line is a ployline that has been selected as the boundary and the outside of the boundary was chosen for the “side” to be trimmed.