Ago and before, after and since. They all have to do with time, and as paired above, they’re often confused. Let’s clear up that confusion.

First, the easy ones, ago and before. Years ago, I noticed my German friends almost always said before when they mean ago, possibly because the German word is the same for both: vor. (Similarly, many languages have a single word that means both do and make. In English, which you use when is idiomatic.) Since then, I’ve heard the same mistake from speakers of many other languages, possibly for the same reason.

In English, if you say something happened seven years before, it means seven years before some other event in the past. If you mean it happened seven years before the present, say seven years ago.

Here’s an example from a December J-school graduate:

All over the world, millions of people think governments are run by a reptilian alien race from Sirius star, who conquered the world 1,000 years before.

Since the main verb of the sentence — think — is in present tense, before needs to be ago. But suppose the writer had been talking about a previous time period:

In the 19th century, millions of people thought governments were run by a reptilian alien race from Sirius star, who had conquered the world 1,000 years before.

He would have been correct.

Before can be either an adverb, as in that sentence, or a preposition:

A reptilian alien race from Sirius star conquered the world 1,000 years before the Industrial Revolution.

But ago is always an adverb; it can never take an object.

After and since are more complicated because using them correctly  involves past tenses. After generally refers to a particular point in time that is now past and thus takes simple past. Since indicates a span of time — starting at some point in the past and continuing to the present — and generally needs to be matched with the tense that also indicates a span of time, namely present perfect. (I keep saying “generally” because, as I tell students almost daily, no rule in English applies 100 percent of the time. There’s always an exception.) After and since provide clues to which tense is correct, but sometimes the right choice depends on meaning.

Some examples from  past, present and maybe future J-school students:

Achin is a district in the eastern provine of Nangarhar, where ISIS holds a wide swath since early 2015.

The writer meant ISIS has held. The action of holding the territory began in 2015 and continued at the time he was writing.

Since my return to Brazil, my work has focused on how Brazil is seen in the world and the impact of Latin America’s biggest economy on the region and beyond.

From the same writer:

This image contrasts with the low self-esteem that is widespread in Brazil after a corruption scandal dragged the country to its worst economic crisis in 2015.

Is self-esteem still low? If so, the writer should have said . . . the low self-esteem that has been widespread in Brazil since a corruption scandal . . . But if it was a temporary condition that has now ended, she meant it was widespread after the scandal.

After Iran’s Islamic revolution in 1979, hundreds of thousands who faced persecution in their country have resettled in the U.S.

This sentence could also go either way — but not as written. Either those refugees resettled after the revolution (but no more are coming now), or they have resettled since the revolution (more are still coming).

A study published Wednesday by the Hamilton Project at the Brookings Institution points out the decrease in start-up creation as a trend in the U.S. economy since the 1980s until 2015.

This writer meant from the 1980s until 2015. Yes, the decrease occurred over a span of time, but that period is now over and since no longer works.

And an applicant for the Class of 2019 wrote:

I can only say that journalism is what I want to do since around a year.

Both the since and the tense are incorrect. A year is a span of time, so it calls for present perfect. But since refers back to a more or less particular point in time, not the entire span. Two possible fixes:

I can only say that journalism is what I have wanted to do since around a year ago.

Or, better:

I can only say that journalism is what I have wanted to do for around a year.