I read a book back in the 90s about a boy who meets a girl on an alien planet, and I think he crash landed there. The cover had green hills/mountains, blue sky and the boy and girl on it (not an 80s style). It was either the second or first book in a series. It wasn't a big book, because it was part of a trilogy or series. There may have been advanced technology, and I think it was either an Australian or Canadian female author.
Not a lot to go on, but it puts me in mind of Monica Hughes' The Keeper of the Isis Light.
My first thought was the People series by Zenna Henderson. I was unable to find a matching cover but part of the plot involves a ship coming from a safe planet to rescue those aliens (basically humans with psychic powers) who had crash landed on Earth. This gives you both the original Earth crash as well as the controlled but rough landing of the rescue ship. On the ship is a girl who sneaks out to experience Earth guided by a boy born since the original crash. The psychic abilities are developed enough to resemble advanced technology in some cases.
From Project Gutenberg:
Most of her stories emphasize the theme of being different and the dangers therein, exemplified by the phrase "Different is dead". They often feature children or young people. Most are part of her series concerning the history of "The People", humanoid beings from a faraway planet who are forced to emigrate to (among other places) Earth when their home world is destroyed by a natural disaster. Scattered mostly throughout the American Southwest during their landing before 1900, they are set apart by their desire to preserve their home culture, including their religious and spiritual beliefs. Their unusual abilities ("Gifts") include telepathy, telekinesis, prophecy, and healing, which they call the "Signs and Persuasions". The stories describe groups of The People, as well as lonely isolated individuals, most often as they attempt to find communities and remain distinct in a world that does not understand them.
While it is never explicitly spelled out in the stories, it is mentioned that very early in the history of The People, they had an expansive, colonial phase, and spaceflight had to be Remembered when the destruction of Home was prophesied, so Earth's humanity may well be their distant cousins. This would explain the similarity between the two species.
Beginning with "Ararat" (1952), Henderson's The People stories appeared in magazines and anthologies, as well as the novelized Pilgrimage: The Book of the People (1961) and The People: No Different Flesh (1966). Other volumes include The People Collection (1991) and Ingathering: The Complete People Stories (1995)
My first thought was Welcome to Mars! by James Blish. I read it as a child and didn't really understand it (too young), but I remember it well.
Dolph Lundgren is a precocious teen who builds his own spaceship in his garden shed. He flies to Mars in it and crash-lands. He lives there for some time, sustained by eating the fungus and lichen that he finds there. Then his girlfriend somehow manages to follow him and crash-lands on Mars right next to where Dolph did. I can't remember how it goes from there, but there's something about her time-of-month moodiness which was beyond my experience and understanding when I read it.
Dolph had found the secret of anti-gravity and now the solar system was his to explore. In his homemade spaceship he soared through the star-studded blackness of outer space. It was all systems go until the power tubes burned out during the landing on Mars. Dolph was now the first man marooned on a strange planet...
What about the Children of the Star trilogy by Sylvia Louise Engdahl 1 This Star Shall Abide (1972) also titled Heritage of the Star (1973) 2 Beyond the Tomorrow Mountains (1973) 3 The Doors of the Universe (1981)
Noren is a rebellious adolescent in a primitive village that depends on rare technology controlled by a priesthood. He thinks they just want to maintain control and there is no purpose behind the rituals and becomes a heretic. Eventually he makes his way to the city where priests and technicians lives, recants, falls in love with a technician, and becomes a priest himself.
That would give you the a boy on a different planet who falls in love with a girl from a more technologically developed city that is beyond mountains. It has been a while since I read them but he does crash land at some point while serving as a technician (I think it is the second book) because I distinctly remember he attempts to calculate how much of the toxic water he can drink on top of his defiant consumption as a heretic and still be able to have undamaged children
Link to a different cover than shown above https://www.risingshadow.net/library/book/7984-beyond-the-tomorrow-mountains
You may be talking about Star Hatchling (1995) by Margaret Bechard.
I don’t believe it is in a series, but it sounds similar to what I remember. There are two different covers of said book. Hope this helps!