The one idea that ties it together
Controlled airspace exists to separate traffic, and the busier the airport, the more separation it needs. That single idea explains almost every rule. Class B wraps the busiest airports in the country, Class C the moderately busy ones with radar, Class D the towered fields, and Class E and G fill in everything else.
Once you stop seeing five unrelated sets of rules and start seeing one scale of busyness, the entry requirements and weather minimums fall into place because they all move in the same direction.
Entry requirements, from most to least demanding
Class B (the upside-down wedding cake): you need an explicit ATC clearance — the words 'cleared into the Bravo' — plus a Mode C transponder and, in most Bravo airspace, ADS-B Out. A private pilot can operate in Bravo; a student pilot needs a specific endorsement and is barred from a handful of the busiest ones.
Class C: establish two-way radio communication and carry a Mode C transponder with ADS-B Out. The magic phrase is that ATC must say your call sign back to you — that's two-way communication established. No formal clearance required.
Class D: two-way radio communication established before entering, and that's it. A tower, but no radar separation guarantee for VFR traffic.
Class E: controlled airspace with no entry requirement for VFR traffic in visual conditions. It's where most of your en route cross-country flying happens above 1,200 feet AGL.
Class G: uncontrolled. No ATC, no entry requirement — but the weather minimums actually get stricter in some respects because you're on your own for separation.
The weather minimums that trip people up
In Class B, the cloud rule is simply 'clear of clouds' with 3 statute miles visibility — because ATC is separating everyone, you just need to see and be seen. Everywhere else the standard VFR rule is 3 statute miles and the 'three-one-two-thousand' clearances: 500 feet below, 1,000 feet above, 2,000 feet horizontal from clouds.
Class G is the exception that rewards careful reading. Below 1,200 feet AGL during the day you can fly with just 1 statute mile and clear of clouds, but at night, and as you climb higher or approach 10,000 feet MSL, the numbers ratchet up. Examiners point at a spot on the chart, ask for the class and the minimums, and watch you reason it out.
The reliable way to make this stick is repetition against a real chart: pick a point, name the class, state the entry rule and the minimums, then check yourself. Do it twenty times in a mock oral and the headache is gone for good.