For, since, and ago: why one Spanish word breaks three English rules

You say I live here since three years and people understand you, but something grates. You say I have arrived two weeks ago and it grates again. This isn't your vocabulary or your level: in Spanish you say hace tres años, desde hace tres años, and desde 2019 without thinking, and your brain routes all three to the same place.

English cuts that territory into three pieces that never touch. For measures how long something lasts. Since marks the point where it started. Ago counts backward from today to a moment that's already closed. Before you open your mouth, you only have to answer one question: are you saying how long, starting when, or how far back? That question resolves most of your errors, and the problem isn't a small one: in a corpus of 304 essays by Spanish university students with 15,850 hand-tagged errors, prepositions account for 14% of all grammar mistakes.

For measures a period, since marks a point#

The base rule fits in two lines from the Cambridge Dictionary: for goes with a period of time, since goes with a point in time. They've lived in Oxford for a couple of months versus They've lived in Oxford since 2004. Same fact, measured two different ways.

The mistake shows up when Spanish hands you desde hace, which sounds like since and means for. Cambridge marks the result wrong without qualification: it's She's been on the phone for hours, never She's been on the phone since hours. If what follows is a quantity (three years, two weeks, hours), you need for. If it's a date, a day, or a time on the clock, you need since.

Wrong: I work here since three years. Right: I've worked here for three years. Why: "three years" is an amount of time, not a starting point. And because the situation is still alive today, the verb takes the present perfect. Real life: a job interview in Dublin, the recruiter asks how long have you been in the role?, and your answer comes out traced from Spanish in the exact sentence she'll remember.

Ago counts backward and shuts the door#

Ago goes after the time expression, never before it. Cambridge spells it out with its own counterexample: They arrived in Athens six weeks ago, not They arrived in Athens ago six weeks. Spanish puts hace in front (hace seis semanas), and that habit is what betrays you.

Ago also refuses to share a sentence with the present perfect. The Cambridge rule is blunt: "We normally use ago with the past simple. We don't use it with the present perfect." Their example is exactly the mistake Spanish speakers make: I received his letter four days ago, never I have received his letter four days ago. Ago pins the event in a finished past, and a finished past won't take a tense that reaches into the present.

Wrong: I have arrived to Toronto two weeks ago. Right: I arrived in Toronto two weeks ago. Why: ago closes the moment off in the past, so the verb goes in the past simple. And arrive takes in with cities, not to. Real life: a clinic front desk in Toronto, the receptionist asks how long you've been in the country, and you answer with a present perfect that makes her repeat the question.

Ago and since don't fit in the same sentence either. It's the quietest of the three clashes, and Oxford flags it as a straight usage error: "It is not correct to use 'since' in this sentence: It's not long ago since they met". And when what you mean is how long something lasted, the word isn't ago, it's for.

The mistake Cambridge has on file: I'm here since 4 o'clock#

When you use since, the verb almost never sits in the present. Cambridge catalogs this exact case on its typical errors page: don't use the present simple or the present continuous before since when you're talking about a specific time up to now. It's I've been here since 4 o'clock, never I'm here since 4 o'clock.

Since opens a line that starts in the past and arrives alive at this second, and English marks that line with the present perfect. It's the mirror image of ago. The present simple describes only the now, with no history behind it, which is why it collides with since.

Wrong: I'm waiting since two hours. Right: I've been waiting for two hours. Why: both pieces fail at once here. "Two hours" is a quantity, so it takes for; and the wait started earlier and continues, so it takes the present perfect. Real life: the waiting room of an auto shop in Phoenix, your car still up on the lift, and the sentence you finally complain with contains both errors back to back.

Why Spanish pushes you straight into this#

Your first language doesn't interfere by accident: it measures time far more than English does. Javier Valenzuela and Daniel Alcaraz Carrión compared corpora in both languages and found duration expressions appearing 1.56 times per million words in English against 67.04 in Spanish, roughly forty times more. Their conclusion is direct: "sequential expressions … and duration expressions … are much more frequent in Spanish." You arrive with the habit of saying how long everything lasts, and English makes you pick the exact piece every time.

That transfer from Spanish has a technical name. Terence Odlin, who wrote the standard reference on it, defines it this way: "The phrase crosslinguistic influence (CLI) is roughly synonymous with other terms, most notably language transfer and interference, in that all refer to the influence of one language upon another". You're not slipping up out of carelessness. You're slipping up because a structure that works perfectly in Spanish already lives in your head.

These errors don't clear up as your level rises. In that Spanish university corpus, CEFR level shaped which error types dominate less than the researchers expected: prepositions are still there at B2 and C1. Another count puts them at 29% of all errors from 53 intermediate and advanced students. Martin Chodorow and Joel Tetreault, who built automatic detectors for these mistakes at ETS, put it plainly: "preposition usage is one of the most difficult aspects of English grammar for non-native speakers to master".

It gets fixed in chunks, not with rules#

The rule gives you the right answer; repetition gives you the speed. For three years, since 2019, and two weeks ago get stored as whole pieces, the same way you store good morning without parsing anything. Once the chunk is automatic, the choice stops being a decision and turns into a reflex.

The real gap is the half second of hesitation, not the rule. Could you say "I've worked here for three years" out loud right now, without stalling to pick between for and since? That half second is the distance between knowing the rule and using it, and it's exactly what Tabla trains: breaking a long sentence into small pieces and drilling them out loud until they come back together whole.

You started at your company in March 2023 and you're still there. A client asks how long you've been with them. Say both versions: the starting point and the duration.

With the starting point: I've worked here since March 2023. With the duration: I've worked here for over three years. Both take the present perfect because the situation is still open today. If you had left that company instead, the sentence would shift to the past simple with ago: I left that job two years ago.

The next time you're about to say hace in English, stop for half a second and answer the question before you pick the word: quantity, point, or closed moment? Once three or four of your own sentences are automatic, the hesitation disappears for good.

Try this today: record thirty seconds of yourself saying how long you've been at your job, in your city, and with your current phone, then listen back for the preposition that made you stall.