"Because" introduces the MAIN cause or a new reason; "since" and "as" introduce SECONDARY or KNOWN reasons (which the listener already knows or takes for granted). Also, "since" can have two meanings: causal AND temporal.
SINCE and AS as causal conjunctions: they introduce reasons the listener already KNOWS or that are ASSUMED — the causal information is background, not the central point. "Since you are already familiar with the project, I will skip the introduction" (the cause is known and secondary). "As the meeting has been postponed, we have time to revise the proposal" (the cause is background information). DIFFERENCE BETWEEN SINCE AND AS: interchangeable in causal use. SINCE is slightly more formal in written English. AS is also common in spoken English. WARNING: SINCE has dual meaning (causal and temporal: "since 2020" = from 2020 to now). When there is ambiguity, use BECAUSE or AS to avoid confusion.
Since the board has already approved the revised budget, the procurement team can proceed with the vendor selection process without requiring additional sign-off.
As the primary contact for this account is on extended leave, all urgent queries should be directed to the deputy relationship manager until further notice.
Since both parties have signed the non-disclosure agreement, we can now share the detailed financial projections that underpin the proposed transaction structure.
BECAUSE: the primary causal conjunction. Introduces the MAIN cause, a NEW reason the listener does not know, or the direct answer to "why?" BECAUSE can go at the start or middle of the sentence. "She was promoted because she consistently exceeded her targets" (cause is the main/new information). BECAUSE in direct answers: "Why was the launch delayed?" / "Because the regulatory approval was not granted in time" (direct answer to "why?" = always BECAUSE). BECAUSE in emphatic cause clauses: "The acquisition failed, not because of financial issues, but because of a fundamental cultural mismatch." BECAUSE = cause is NEW, IMPORTANT, or the MAIN POINT.
The expansion into the Southeast Asian market was delayed primarily because the regulatory landscape in several key countries proved significantly more complex than the initial feasibility study had anticipated.
The proposed partnership was ultimately declined because it would have required the company to share proprietary technology that is central to its long-term competitive advantage.
The project was considered a success not because it was delivered on time and within budget, but because it fundamentally changed how the organization approaches customer data management.
Common mistake
THE MOST COMMON ERROR: "I am late because I am stuck in traffic" vs "Since I am stuck in traffic, I will be late" — both are correct, but the choice of connector changes emphasis. "Since" at start = cause is known background. "Because" in the middle = cause is the new information. Second error: using SINCE when there is temporal/causal ambiguity. "Since he left, things have changed" = AMBIGUOUS (since he left = temporal? or because he left = causal?). Solutions: "As he left, things changed" (causal, clear) or "Since he left in 2023, things have changed" (temporal with clear reference). Third: "As" in causal sense at the start can be confused with "as" of simultaneity ("As he was reading..."). Context clarifies, but use "since" if in doubt.
Trick to remember it
THE "WHY?" TEST: if the sentence directly answers the question "why?" = use BECAUSE. If the reason is KNOWN or SECONDARY = use SINCE or AS. The EMPHASIS TEST: is the cause the most important information in the sentence? = BECAUSE (emphasis on the cause). Is the result the most important information? = SINCE/AS (the cause is only background). SINCE at sentence start = the cause is already shared information with the listener. BECAUSE in any position = cause is central. Warning about SINCE: if you can substitute "since" with "from that moment until now," it is TEMPORAL. If you can substitute with "given that/because," it is CAUSAL.
Quick rule
Because = NEW or MAIN cause (answers "why?"). Since/As = KNOWN or SECONDARY cause (shared background). Since double meaning: causal ("given that") + temporal ("from that time"). With ambiguity = avoid SINCE, use AS or BECAUSE. Position: SINCE/AS frequently at the start. BECAUSE at start or in the middle.
Did you know?
The distinction between BECAUSE, SINCE, and AS in modern English reflects a hierarchy of information that linguists call "information structure" or "information packaging." Human languages have systematic ways of marking which information is "new" (foregrounded, important) and which information is "given" or "background" (already known, secondary). In English, BECAUSE marks the cause as FOREGROUND information, while SINCE and AS treat it as BACKGROUND information. This distinction exists in Spanish with "porque" (emphatic cause) vs "ya que/puesto que/dado que" (known background cause), but Spanish-speaking learners often translate everything as "because" without exploiting the nuances of SINCE and AS, making their English less natural and sophisticated in formal registers.
Exercise 1 · Pick the right one
The quarterly results were disappointing _____ the stronger-than-expected performance in the domestic market was offset by significant foreign exchange headwinds in three key international regions.
Exercise 2 · Pick the right one
_____ the technical specifications for the new platform have already been reviewed and approved by all relevant stakeholders, the development phase can now begin immediately.
Exercise 3 · Form the word
Because = NEW or MAIN cause (answers "why?"). Since/As = KNOWN or SECONDARY cause (shared background). Since double meaning: causal ("given that") + temporal ("from that time"). With ambiguity = avoid SINCE, use AS or BECAUSE. Position: SINCE/AS frequently at the start. BECAUSE at start or in the middle.
THE MOST COMMON ERROR: "I am late because I am stuck in traffic" vs "Since I am stuck in traffic, I will be late" — both are correct, but the choice of connector changes emphasis. "Since" at start = cause is known background. "Because" in the middle = cause is the new information. Second error: using SINCE when there is temporal/causal ambiguity. "Since he left, things have changed" = AMBIGUOUS (since he left = temporal? or because he left = causal?). Solutions: "As he left, things changed" (causal, clear) or "Since he left in 2023, things have changed" (temporal with clear reference). Third: "As" in causal sense at the start can be confused with "as" of simultaneity ("As he was reading..."). Context clarifies, but use "since" if in doubt.
THE "WHY?" TEST: if the sentence directly answers the question "why?" = use BECAUSE. If the reason is KNOWN or SECONDARY = use SINCE or AS. The EMPHASIS TEST: is the cause the most important information in the sentence? = BECAUSE (emphasis on the cause). Is the result the most important information? = SINCE/AS (the cause is only background). SINCE at sentence start = the cause is already shared information with the listener. BECAUSE in any position = cause is central. Warning about SINCE: if you can substitute "since" with "from that moment until now," it is TEMPORAL. If you can substitute with "given that/because," it is CAUSAL.
Bryan López
Founder of SpeakUP Academy