There is a difference between believing and knowing, and even as the warm, welcoming sun beats down on him Tuesday afternoon, Ryan Zimmerman could feel that gap. He fielded grounders and buzzed throws across the Space Coast Stadium diamond from third base. Some balls sailed. Some bounced. Most burrowed into a coach’s glove. Strength has returned to his shoulder. He believes he will revive his old ability. But he doesn’t know. How could he?

“For me,” Zimmerman said, “I need to have a year like I used to have.”

On the day the Washington Nationals required position players to report, Zimmerman took the field for the first time and joined about 20 teammates in an informal workout. He remained part of the franchise’s heart. He left the black Rawlings first baseman’s mitt in his locker and stayed on the familiar side of the diamond. But he began a spring in which he will become a part-time first baseman and try to replicate the final portion of his 2013 season.

Doubt shadowed Zimmerman last year. He arrived at spring training still recovering from offseason shoulder surgery. His mechanics deteriorated and he made 16 throwing errors. In May, he wondered if the Nationals would shift him across the diamond or bench him. Into mid-August, Zimmerman was hitting .266 with little power.

On Sunday, hitting coach Rick Schu walked up to Zimmerman in the clubhouse and extended a fist. “Every day like September,” Schu said. It was a reminder of what he could be. He blasted 11 home runs in the final month. In the field, he made diving plays and accurate, clothesline throws.

The Nationals' pitcher Stephen Strasburg discussed the state of his arm, lessons learned from last season and what's to come for the Washington team during a break from spring training in Viera, Florida. (Whitney Leaming/The Washington Post)

September brought back confidence. But only one thing can provide Zimmerman certainty.

“Play third base like I know I’m capable of and like people expect me to play,” Zimmerman said. “Then once I go a year doing it and being consistent, that’s when you can really say the shoulder’s fine. Until we go through this year, I don’t think you really know.”

Zimmerman also will learn a new position this spring. Manager Matt Williams met with him in December and notified Zimmerman he wanted him to learn first base. In the spring, Zimmerman will play third base “99 percent of the time,” Williams said. But in early work and extra work, Zimmerman will learn the nuances of first base and view the game from the opposite side of the field.

Williams wants the option to play Zimmerman at first base to stack the lineup full of right-handed hitters against tough left-handed starters. In that scenario, Adam LaRoche would sit, Anthony Rendon could play third and Danny Espinosa would play second.

“He understands that he’s our third baseman,” Williams said. “It’s on a temporary basis if we want to gain an advantage with our lineup. Nothing further than that.”

From their initial conversation, Zimmerman approved. “He wants to win,”’ Williams said. “If he helps us win by playing some first base, that’s great by him, and certainly great by me. He’s not worried about that.”

“I don’t know if I’m ready for 65-70 games over there, but you never know,” Zimmerman said. “With the way I finished [last season] over there, the way my arm feels now, I think I can help this team win more games at third base. But if there’s an opportunity for me to play 10-15 games at first base and it helps us win some of those games, then I’m down to help out any way I can.”

Zimmerman has not played first base since Little League, when “if you can catch the ball, you play first base.” Zimmerman plans to take groundballs at first base during batting practice, studying how the ball comes off the bats differently. He will work after practice to learn how to hold runners, catch pickoff throws and handle relays from the outfield. At 29, in his 10th season, Zimmerman will receive a new baseball education.

“No position on the field is easy at this level,” Zimmerman said. “But for some reason, everyone just thinks that’s where they put the adult softball guy and he can play big league first base, which is not the case. I’ll work at it. He made it very clear, and I respect him for that, that he doesn’t want me to feel uncomfortable or put me in any situation to fail.”

Zimmerman bought a first baseman’s mitt this winter. He’s played catch with it a few times, and it didn’t feel too weird. But he does not want to wear it permanently. “It feels all right,” Zimmerman said. “It’s a lot easier over there than third base, but I think I can still . . . if I can help the team at third, I’d like to stay there for a majority of the time for as long as I can. But we’ll see. Things happen.”

For Zimmerman to stay at third base for the remainder of his career, or at least the next few seasons, he will have to prove the start of last season was a surgery-related aberration. This winter helped. For the first time in three years, Zimmerman had no offseason surgery and he could begin his offseason workouts Nov. 1, on schedule. Last year, after the operation to heal fraying in his labrum, he couldn’t lift weights until January.

“My shoulder’s fine,” Zimmerman said. “It’s just a matter of going out there and playing like the last six weeks, seven weeks, whatever, of last season where I played good.”

That would be a year like he used to have. That would let him stop believing and let him know he was back.