On all the other sites that I have read, you conjugate the verb in the correct tense when using desde. Can you please clarify this for me? I read in the comments that the tendency is to use the present tense, but why do none of the other sites say this?
As we don't know which other resources/ sites you've seen I'm not sure, but the most natural way to say that you've done/haven't done something since (time) is by using the present tense. You may see it occasionally using the perfect tense but it's not the most general usage.
I had a quick look at other resources that coincide with this usage (present) and here are two other examples:
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