Ahmad Shah Massoud was fairly important to the war effort. While he did not have much clout in the majority of the country, he dominated the Tajik north, particularly the area around the Panjshir Valley. This area contained important supply lines for the Soviet forces, so the constant harassment by Massoud's forces actually led to KGB higher ups signing a temporary peace deal with Massoud in 1983. Massoud used this deal to safely bulk up and train his forces, which then led him to continue harassing Soviet forces a few years later (I don't have the dates on the top of my head.) Massoud was also a leading member of the coalition that captured Kabul in 1992.

What motivated him differently from others?

Many of the other poweful mujihadeen commanders (Hekmatyar, Sayyaf, Haqqani) were hardcore jihadis from the Pashtun highlands. Massoud was more socially liberal, and he fought more for his Tajik people and for Afghan nationalism than for religion. After the war ended, Massoud repeatedly entered coalitions against jihadi warlords, eventually creating the Northern Alliance to fight the Taliban. This was not very successful though, as the Taliban pretty much dominated the Pashtun areas of Afghanistan while Massoud and his allies were left claiming a (shrinking) piece of territory composed mainly of Uzbeks and Tajiks until the American air war began in late 2001. Massoud was killed on September 9, 2001 as Northern Alliance lines were crumbling in the face of the stronger Taliban force.

how was he important in the formation of modern Afghanistan?

Unfortunately, not all that important. as mentioned above, Massoud was killed prior to the American war in Afghanistan. When the war began, the American special forces and the CIA coordinated closely with Northern Alliance forces to guide airstrikes against Taliban positions, ultimately routing them from most of the country before the ground invasion by American troops even began.

When it came time to form a political future for Afghanistan though, Massoud's people were more or less sidelined. The Pentagon supported bolstering Karzai (Pashtun) as the leader of Afghanistan, while some other factions in the USG (mainly the CIA) supported handing the country to the Northern Alliance. Without Massoud though, the most recognizable figure of the northern alliance was General Dostum (current VP of Afghanistan), but Dostum had actually sided with the USSR against the mujihadeen during the war, so he didn't really have any credibility as an Afghan leader. The loya jirga agreed that Karzai would be president of the country, and his relations with the US more or less dominated the politics of the country since.

There is a fair argument though, that Massoud's legacy has played a defining role in the direction of the country. He is still considered a hero by many, and many Afghans who don't support the Taliban still carry pictures of him and view him as something of a spiritual leader.