主要（的）是grammar point · tier 2 · 主要（的）是 — the main one is (single out the chief reason/factor)
Singles out the principal reason, purpose, or factor from several, foregrounding it as the dominant one while smaller ones stay implied.
字源 FORM what the parts do
主 is a lamp-flame on its stand, the one point a room turns toward, borrowed here for chief. 要 is the waist, the body's middle, extended to essential. Together 主要 names the main one. 的 is optional and only smooths the join. 是 equates: it points at the principal item and sets it down as the answer.
故事 STORY a scene to remember it by
Several reasons stand in a row; one is pulled forward into the lamplight and the rest fall back into the dark.
字源記憶法
框 · Frame
[setup / several factors], 主要（的）是 [the chief reason/purpose/factor]
觸 · Trigger
You name a situation with more than one reason behind it, then push the dominant one to the front.
序 · The move
1Lay out the situation first — the meeting, the choice, the result — so there is a field for the main reason to stand out from.Is there more than one factor in play? 主要 only earns its weight when smaller reasons exist behind it.
2Put 主要（的）是 at the head of the clause that gives the chief reason or purpose.Drop 主要 and the clause still states a reason; 主要 only ranks it as the dominant one, not the sole one.
3Let 是 carry whatever follows — a verb phrase (主要是想…), a noun (主要是錢的問題), or a clause.是 here equates, not 'to be true'. The thing after it is the answer to 'which one matters most', not a yes/no fact.
例 · Examples
1今天請大家來開會，主要是zhǔyào shì — the main thing is想了解liǎojiě — to understand, grasp大家對這個問題的看法kànfǎ — view, opinion。
I've asked you all here for the meeting today; the main reason is that I want to understand everyone's views on this issue.
因為…所以… draws the full cause-to-effect line. 主要（的）是 ranks among several causes and lifts the biggest one forward, leaving the others implied. One reason out of many vs the whole causal chain.
是…的 frames one already-known detail of a past event (time, place, agent) between 是 and 的. 主要（的）是 has no closing 的 frame — its 的 is optional and sits before 是, and it ranks reasons rather than spotlighting a past circumstance.
只是 says the reason is small or sole ('it's only that…'). 主要是 says it is the big one among several. Downplay vs rank-to-the-top — opposite weight.
✗ 我來開會，主要的因為想了解大家 (主要 cannot take 因為 as its tail) → ✓ 我來開會，主要是想了解大家
✗ 我不去，主要是下雨。沒有別的原因 (claiming the sole reason contradicts 主要) → ✓ 我不去，就是因為下雨 / 我不去主要是下雨，也有點累
✗ 他是主要來看你的 (主要 as adverb sits before the focus clause, not inside 是…的) → ✓ 他來，主要是想看你
English 'mainly' / 'the main thing is' slots in freely as an adverb anywhere. In Chinese 主要 leads the clause and reaches for its own 是 to introduce the reason; it does not float to the end or stand alone without 是 before a reason clause.