Before beginning my self-guided course in Trees of Urban Massachusetts, I could identify a shamefully low number of trees.
Oak, identified by leaf shape and acorns
Maple, by Canadian flag leaf shape and helicopter seeds (although I've since learned that a number of other trees have similar leaf shapes)
Japanese maple, by small size and pointy, pot-like red leaves
Birch, by white bark
Weeping willow, by weeping shape (although I've since learned that I called any weeping tree a willow)
"Evergreen", or Christmas trees, a general class of any vaguely triangle-shaped tree with yearlong green needles or scales, and some sort of cones
Certain fruit trees as long as they are actually bearing recognizable fruit, like apples, pears, or lemons. I identified most of the common, urban trees bearing deep red, berry-like fruit as crabapples, which I still think is usually correct.
That's basically it. Here are the new trees I've added in the last few months. I'm fairly confident in my identifications of these, since I've noticed the relevant features even outside the Arboretum and away from the helpful plaques.
Gingko, by leaf shape (can't believe I never noticed this very distinct shape before) and stinky fruit
Beech, by the nuts (there's a giant one right outside my house and for weeks in fall, the outside stairs are covered in pointy, burr-like nut shells)
Linden, mostly by leaf shape, and sort of a default since it's up there with oak and maple as most common tree in the city.
Sycamore, by camouflage-like bark and seed balls (I used to misidentify this as a maple because the leaves are similar)
Tuliptree, by pointy bracts in winter (I also used to identify this as a maple)
Sweetgum, by pointy, maple-meets-pot leaf shape (I also used to identify this as a maple)
Sassafras, by dinosaur footprint leaf shape
Magnolia, by pussy-willow-like fuzzies in winter
Dawn redwood, by the reddish bark color and red tassels
Hawthorn, by the long, creepy thorns; creepy, meandering shape; and creepy bright-red berries (although sometimes I misidentify other creepy trees as hawthorns)
Also, I can narrow down the evergreen identification to pine vs. fir/spruce/hemlock vs. cypress/sequoia, and can sometimes make more specific identifications. Norway spruce is the weepy one.
I'm ready for my first exam!