if Statement

if / elseif / else

if condition then
    # body
elseif otherCondition then
    # body
else
    # body
end

The then keyword is required after each condition. The block is closed with end. The elseif and else clauses are optional, and any number of elseif clauses may appear.

elseif vs else if

These are both valid but semantically different.

elseif (one word) is part of a single if statement and requires one end:

if score >= 90 then
    grade = "A"
elseif score >= 80 then
    grade = "B"
else
    grade = "F"
end

else if (two words) creates a nested if inside an else block. Each if requires its own end:

if score >= 90 then
    grade = "A"
else
    if score >= 80 then
        grade = "B"
    else
        grade = "F"
    end       # closes inner if
end           # closes outer if

Use elseif for straightforward conditional chains. Use else if only when you need to declare variables or execute statements in the else block before the nested condition.