love - ashaya
This is used the same way the English word is.
beloved - ashayam or k'diwa
a beloved person; dear - tal-kam
Ashayam is the general term that two people in a relationship might use for each other. It is derived from the word ashaya.
K'diwa carries more emotional connotation and is barely used for that reason. It stands for very deep affection and should be used between married partners only. It is a shortened form of the ancient word k'hat'n'dlawa, which means “half of each other’s heart and soul.”
Tal-kam or the suffix -kam in general is more commonly used to affectionately address a younger person. Saavik, for example, is (in private) usually addressed as Saavik-kam by Spock.
friend - fainusu
companion - besu
lover, a loving person - ashausu
friend, lover, life-long companion - t'hai'la
Fainusu, literally “known-person”, would be used for regular friends and acquaintances, but not as a term of endearment.
Besu, the companion, just means a person who travels with or accompanies another.
Ashausu is a more technical term for lover, literally “love-person” and means any person who loves - for example, ka-ashausu means homosexual.
Then there’s t'hai'la (more commonly, but incorrectly spelled t'hy'la), which always raises some controversy, mostly because it also translates as brother. However, this absolutely does not mean biological brother, which is sa-kai in Vulcan. T'hai'la is an intimate term of endearment that is probably best translated as “soulmate”. It implies a very deep friendship and connection that is not necessarily platonic.
I hope this helped and I apologise that it took so long
Sochya eh dif