Stephen Strasburg allowed five runs on eight hits, including two home runs, in four innings against the Giants on Sunday. (Luis M. Alvarez/AP)

The Nationals completed a spectacular nine-win, one-loss homestand on Sunday to conclude a two-week stretch in which they went 12-1 and probably won the National League East crown. Two weeks ago when they left Atlanta, they led by 31 / 2 games. Now, after chopping two weeks off the schedule, they lead by eight full games and have a 96.6 percent chance of winning their division.

Their final home game before a tough nine-game road trip brought two facts into clear focus. First, they’re really good: the Nats went from walk-off mode to “stomp-on” mode to crush San Francisco under an 18-hit, 33-total base onslaught. All the slugging was condensed into their last five at-bats after Stephen Strasburg pitched them into a 5-0 hole, allowed two homers and left after four innings.

That pitching performance underlined the second fact: Strasburg should no longer be a lock to start Game 1 of a playoff series. He’s a candidate. As recently as five days ago he pitched the second of back-to-back games as impressive as any he’s produced. And he did it by focusing on a method, “pound ’em inside,” which is ancient, valid and suits his style and personality. There was optimism among teammates that he’d found a key, turned a corner — pick your pet phrase.

But Sunday’s work, including a home run to the first man he faced, underlined Strasburg’s problem. The most basic pitching skill is fastball command. Can you throw it where you want it? Not approximately, but precisely so that it would hit some part of the catcher’s glove if he never moved at all? In about two-thirds of his starts, Strasburg can. Then he’s dominant. The plate then opens up for his devastating change-up and curveball.

But about one third of the time, he can’t. In 10 of his 28 starts, he’s been fairly ugly, allowing 51 runs, or more than five per game. Sunday was typical of those. And, almost always, the cause — and excuse — cited for his problems is “fastball command” and an inability to adjust, to find a way around that flaw, on his bad days. Some pitchers miss “off the plate.” Strasburg tends to miss “middle-middle,” especially when he flies open in his delivery. His fastball, if it’s aimed inside to a left-hander or outside to a right-hander, tails back over the middle of the plate. Same result: a lot of his 21 gopher balls.

The Post Sports Live crew debates what the odds are that the division-leading Baltimore Orioles and Washington Nationals meet in the World Series. (Post Sports Live/The Washington Post)

“I’m making dumb pitches,” Strasburg said after his start. “You want to challenge them, but at the same time, you got to really focus in on your spot. I wasn’t really doing that today.”

When he tried to get inside on lefties, did his fastball tail back over the plate?

“That’s kind of where it misses sometimes. It’s something I’ve been battling all year. Sometimes, my command inside is really good. Sometimes it comes back over the plate. The biggest thing is, you got to go out there and keep pitching. It’s going to figure itself out.”

Strasburg’s ceiling and the distance the Nats go in October may well be connected. Reliever Drew Storen describes his teammate’s stuff as “beyond dynamite.” Strasburg now leads the NL in strikeouts with 202. So the Nats will cling to the notion of Strasburg as both ace and Game 1 starter until he drives them off it. But days like this show how difficult it sometimes is for hugely talented young pitchers to find their own style, learn to adjust mid-game on the fly and become as great as their promise.

Bert Blyleven and Nolan Ryan, both in the Hall of Fame, were little more than .500 pitchers into their late 20’s. Strasburg, with a career record of 39-29 (.574), is actually ahead of them in that regard. But he resembles them in the years of frustration that they brought to themselves and their teams as they developed.

It’s commonly and casually said that a young hurler is “learning to pitch,” as though it were as straightforward and systematic a task as learning geometry in junior high school. If you go to class and do your homework, it’s hard but can be mastered.

Unfortunately for the Nats and their sometimes puzzled prize student Strasburg, pitching in the majors is not entirely logical, systematic and mastered just by hard work. If it were, Strasburg, who’s diligent, would be a polished ace instead of an inconsistent aspiring one.

Can or should Strasburg be entrusted with a Game 1 start in October, especially on a staff that has two excellent finished-product starters who know exactly what their formula for success demands in two-time all-star Jordan Zimmermann and Doug Fister, both images of pitching maturity? The last five weeks of this season, in which Strasburg will get at least six starts, will be a stage for him to show how much he’s learned to pitch — on poor and middling days, as well as in punch-’em-all-out games.

Pitching isn’t just learning to throw a fine fastball, curve and change-up, or even being able to throw them for strikes. It is the art of pitch selection, even for those with lots of raw stuff, as well as the knack for avoiding the heart of the plate with your mistakes. It’s temperament and tough-spot psychology. Pitching is . . . see, that’s the problem. It’s different for everybody.

It’s Greg Maddux’s swing-back fastball on a left-handed hitter setting up his change-up low and away. It’s the 45-degree tilt of 6-foot-10 Randy Johnson’s delivery making his fastball up and away look almost like his slider down and in to a right-handed hitter until it’s too late for the hitter to adjust.

With the Nats, it’s Craig Stammen, who threw four pitches in four quadrants of the strike zone when he was an unsuccessful starting pitcher, deciding to throw only two pitches — sinker and slider — in only two spots — down and in and down and away.

Whatever it is, Strasburg flirts with finding it, then loses it. He hears lectures about how he has to “throw strike one” and “pitch to contact” to get quick outs or “be mentally tough” when things go haywire behind him or pound ’em inside to open up the outside corner. Amid all the advice, Strasburg hasn’t quite found his way. But he may be close. The Nats certainly think so.

“If you can hit your spots, you can get people out. I think Stephen has established who he is,” Manager Matt Williams said. “He has wonderful talent. He’s like nobody else.

“But everybody has to throw it where they want to. The last two games, he threw it exactly where he wanted to. Today, he didn’t. And if you miss over the middle, anybody can get hit hard.”

All his pitching life, Strasburg never had to adjust to others. Good day or bad, his stuff was so dominant that he struck everybody out, more or less. So for five years, sometimes while recovering from injuries or working on an innings limit, he’s been a gifted thrower gradually turning into a pitcher — and doing it in the majors under the pressure to win immediately in pennant races.

The job isn’t finished yet. As a possible division series approaches in which one, and only one, pitcher is likely to pitch both Games 1 and 5, the Nats will have few tougher questions than deciding whether Strasburg is close enough to the polished product.

On Sunday, and for this entire season, I’d say, “Not quite,” and go another direction. But it isn’t late August that matters. It’s Oct. 1. Between now and final exams, how much will the prize pupil learn?