Samuel is one member of some group. This explains why the verb of the matrix clause is "is". Without the relative clause, the sentence is simply:
One [of the boys] is Samuel.
The original sentence does include a relative clause, and the subject of that clause is the relative pronoun "who". By itself, "who" doesn't have a grammatical number. The relative pronoun must have an antecedent. Whatever "who" references, that is the thing that will have a grammatical number.
There are two possible antecedents: "one" and "boys".
- One [of the boys] [who always gives the correct answer] is Samuel.
- One [of [the boys who always give the correct answer]] is Samuel.
In the first sentence, the phrase "of the boys" and the clause "who always gives the correct answer" each independently modify the subject "one". The "who" refers to "one", and so the verb "gives" agrees with its singular subject.
In the second sentence, the clause "who always give the correct answer" modifies the object "the boys". The "who" refers to "boys", and so the verb "give" agrees with its plural subject.
The example sentence matches this second possibility. Several boys always give the correct answer, and Samuel is a member of this group.
Both possibilities are grammatically sound. Each possibility has a different meaning, so the correctness of the form of the verb "to give" depends on which meaning is intended.
Without further context, the second possibility seems more likely and more useful. "Samuel" is a traditionally masculine name. If the entire prepositional phrase is "of the boys", that hardly counts as useful information about him. However, if the prepositional phrase is "of the boys who always give the correct answer", that certainly counts as useful information about him.
Two teaspoons of sugar is more than enough.
Actually, it's not a partitive construction. Partitives normally require a definite noun phrase, the kind with the determiner "the", as in Some of the sugar is missing, or a genitive pronoun, as in Some of his food was eaten. But in Two teaspoons of sugar, "sugar" is not a definite noun, so this is simply a noun phrase with teaspoons as head and of sugar as complement.
Partitive is intended to denote a part rather the whole thing. Some of his food is a partitive construction in that the partitive his food denotes a quantity and Some of his food denotes a subquantity of that quantity. We understand it to mean Some food from his larger quantity of food was eaten.
Measure phrases like the plural two teaspoons in your example can be conceptualised as referring to a single measure which can override the plural form in determining the form of the verb. Which is why singular "is" is fine.
Best Answer
You're correct in thinking that the singular form of the verb should be used, but the singular past tense of 'be' can be 'I was', 'You were', or 'He/She/It was'. As the verb follows 'you', it should be 'were'.