The Archer from the Fate:SN series is both a great archer and a great sword/knife user. Since he's Archer, you would think that he was an archer first. However, I have been told that apparently,
Archer is the future Shiro.
Since this character learnt swords first why was he the archer and not an Assasin, a Sabre or something else. What did the Archer learn first, the Sword or the bow?
since Shiro started to use swords first
That is incorrect.
This is not clear neither from the anime nor from the VN, but Shiro is highly skilled in archery way before the story starts. This is why he is hanging around archery range early in the story.
Somewhere I read that Shiro was so good, he had to put effort into actually missing the target. He stopped practicing archery because he was bored of succeeding while putting no effort into it.
It was only after Holy Grail War started that he began to actually use swords and copying Archer's style.
Neither. The Archer class servant of the 5th Fuyuki Grail War was a magus before anything else. At his core he is Projection and Reinforcement magecraft, all culminating into his reality marble (an innate bounded field) "Unlimited Blade Works."
As a magus his talents were unremarkable, save for his talent in Reinforcement and Projection. While Emiya Shirou was in his school's archery club, it was short-lived and not very noteworthy. Most of his strength came from him making a contract with the World in order to become a Heroic Spirit in the form of a Counter Guardian after his passing.
His adeptness in melee combat is unusual for a servant of his class, but his Origin is "Sword." It's inevitable that we would use them in one way or another. The twin swords he uses, Kansho and Bakuya are not tied to him as a servant, but was merely something he projected. He utilizes these twins swords because of the innate magic resistance (to deflect Caster's A-ranked spells) they give, in addition to being effective against monsters such as Gorgon form of Rider and the extradimensional abomination summoned by Caster (from Fate/Zero).
Servants are placed under the Archer class based on their Noble Phantasm. The user does not have to be adept at a bow, but any projectile weapons, even firearms. Gilgamesh is placed under Archer based on his Noble Phantasm "Gate of Babylon," which essentially is him launching Noble Phantasms stored in his treasury.
"Unlimited Blade Works" allows allows him to reproduce just about any weapon he's seen (with some exceptions), allowing him to summon an arsenal of weapons as he needs it. His Projection magecraft allows him to create and overload projected Noble Phantasms (the copies are one rank lower than the original). The overloaded Noble Phantasms called Broken Phantasms are increased by one, but are destroyed they are used. Combining Broken Phantasms with a bow allows Archer to nuke the enemy from extreme distances. Think of as having an unlimited quiver of super powerful arrows for just about any situation. Tactics such as these make him suitable to be classified as an Archer class servant.
Additionally, it should be noted that this Archer is not from the same timeline as Emiya Shirou. He is an Emiya Shirou that walked a different path as a nameless hero of justice, who made a contract with the world to be save humanity as a whole as Counter Guardian. But in the end, he was made to clean up after the actions humans by becoming an assassin. To save humanity as a whole, instead of "saving everyone" as he had hoped.
It's possible for him to be summoned as Caster, Berserker, or Assassin, but that's a story for another time.
Archery. Archer's magecraft is remarkably close/based off the 8 steps of Japanese Archery/Kyudo/The way of the bow, and his lack of self identity is the reason why he's good at it. The weapon might have changed (his body instead of the bow, swords instead of arrows), but the steps are the same
Archery is Shirou's basic talent, he didn't even need to learn it to be good at it (as mentioned above, he is way too good for the archery club), sword technique is more like his hobby or inner calling (because of Avalon, made his Origin: Sword) and he is not really talented at it no matter he tries even on his max potential + World-boost, his swordmanship is still below standard close-combat Servants (although he can defensively fight against those "True" melee combatants, such as Saber and Lancer with his True Mind's Eye (spider-sense-like skill)
If he was really intended to win the Holy Grail, he would just sniped every Master and Servants from afar (including Lancer, who has Arrow Evasion skill/blessing or such, but only worked if Lancer could see where the arrow coming from and the arrow had no AoE like Caladbolg II ... on Berserker, he could just snipe Ilya anyways (but for unknown reason, Archer won't or can not kill Ilya, perhaps Archer came from Shirou-Ilya path? lol), and Berserker will run out gas later if there is no Master supplied his Mana), i.e. his meele sword play is more like a handicapped fighter, and not even trying to win on most cases.
So my conclusion based on VN and animes: his granted Sword origin from Avalon gave him an endless craving for Sword techniques, but will never reaches mastery like his Archery does ... also maybe he thinks Archery is a coward skill, and he prefers to fight his opponent bravely like other meele combatants lol
He learned Archery before and he was pretty accurate when shooting, making him Archer material. His archery skills is even better than his magic when he was still Shirou.
Shiro used to be on the Archery varsity team in his school. That's why he is good at using bows. He became good in using swords/knives because of the fighting experiences. mostly when he fought Gilgamesh, which sharpened his sword intuition. Shiro is an archer who can wield blades for short.
