There are two essential prepositions for talking about how long something has been happening with the present perfect (or present perfect continuous) tense. For and since are often confused or used incorrectly, however, so let’s do a quick clarification!

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The preposition since can only be used to reference a point in time, NOT a duration. So you can say since 2001, since September, since last summer, or since Tuesday, but NOT since five days.

Tom and I have been working together on Rachel's English since two thousand twelve.

Caption 4, Exercises - Tongue Flexibility and the N [n] Sound

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In fact, since nineteen sixty-nine, fifteen other rare and endangered species have also been rescued from the brink.

Captions 50-51, BBC Planet Wild - Alien Animals - Part 5

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For, on the other hand, refers to a duration. It doesn’t matter if something has been happening for 20 minutes or for 20 years. 

We've been doing freestyle for a couple of weeks.

Caption 25, Kiteboarding - Sam Light Interview

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I have been working at the company Phonez and More for several months now.

Caption 1, Business English - Difficulties with coworkers and contracts - Part 1

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I've been on this boat for twenty-two years.

Caption 3, Aqua Quest - Boo Boo

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While for can also be used with the simple past tense or future tense, since is always a clear indicator of the present perfect or present perfect continuous (See this newsletter for more information!). 

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Further Learning
On Yabla English, for and since can be found in most videos! There is even one video in which a famous actor actually misuses the word since, which is indicated in the captions with sic (sic erat scriptum, Latin for "thus was it written"). Can you find it?
 

Whenever you see what appears to be a verb in English ending in -ing, you have to be careful as to how you interpret the sentence, as it may wind up that this apparent verb is actually a noun or an adjective! In English, gerunds and present participles are formed by adding -ing to the infinitive form of the verb ("to surf" becomes "surfing"), or for verbs ending in -e, dropping the -e and adding -ing ("to love" becomes "loving"). A gerund is a verb that acts as a noun in sentence. A present participle is a verb that is used to make a verb phrase or an adjective.

Therefore an English verb ending in -ing can either be noun (gerund), an adjective (formed from a present participle), or a verb (a present participle). This all sounds a bit complicated, but if you look at some examples, it's pretty easy to tell the difference!

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I've always loved surfing.

Caption 19, Kiteboarding - Rider Profile - Tom Court

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What does he love? He loves surfing. In the above example, the subject of the sentence is "I," and the object of the sentence is "surfing." Since you can make a noun out of "the surfing" as used here, it is a gerund.
 

We watch a couple of surfing videos.

Caption 26, Kiteboarding - Sam Light Interview

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What kind of videos is he watching? Surfing videos. Here it is clear that "surfing" is an adjective that is modifying the noun "videos."
 

The four of us have just been surfing different spots.

Caption 10, Naish SUP - Aloha Big Island!

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What have they been doing? They have been surfing. In this last example, by pairing the verbs together, you get "have been surfing." This is the verb "to surf" in its form as present participle verb.

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Further Learning
Write down some of your favorite verbs, add -ing to them, following the rules above, and search Yabla English to see them used in a real-world context as either a gerund, adjective, or present participle verb.