The words since and when are subordinating conjunctions — words that connect two clauses together. When they introduce a clause in a Present Simple sentence, the logic changes depending on the context. This page covers two distinct situations where the Present Simple is essential with these words.
This is the most important rule on this page, and the one that causes the most errors for ESL learners. In English, when you use a time conjunction — when, as soon as, before, until, after, once — to refer to a future moment or event, you use the Present Simple in that clause, not the future tense.
Think of it this way: the time clause describes the condition that must be met — and in English, conditions are expressed in the Present Simple, not the future. The future tense (will) goes in the main clause only.
The word since has two meanings in English. One is related to time (since 2010 — used with the Present Perfect). The other meaning is because / given that — it introduces a reason or explanation. In this second meaning, since is followed by the Present Simple when describing an ongoing, general situation that is always true.
✗ I will call you when I will arrive. · Tell her as soon as you will know. · Wait until she will finish.
✓ I will call you when I arrive. · Tell her as soon as you know. · Wait until she finishes.
This is by far the most common error. The time conjunction (when, as soon as, until, before, after, once) already tells the listener this is a future event — you do not need will to say it again. Think of the time clause as setting the condition: it uses the Present Simple. The main clause carries the will. Never put will in both clauses.
✗ Since she is working nights, she sleeps during the day. · Since they are living far away, we rarely see them.
✓ Since she works nights, she sleeps during the day. · Since they live far away, we rarely see them.
When since means because / given that and introduces a permanent situation or general truth, use the Present Simple — not the Continuous. The Present Continuous suggests a temporary situation happening right now. A permanent situation (where someone lives, what someone does for work, a regular fact) always takes the Present Simple.
✗ Since she works here, she has a lot of experience. (ambiguous — could mean "because" or "from the time she started")
✓ Since she works here, she knows the system well. (since = because — Present Simple in both clauses)
✓ Since she started working here, she has gained a lot of experience. (since = from that time — Present Perfect in main clause)
Since as a time expression ("from a point in time until now") is used with the Present Perfect: I have lived here since 2010. This is different from since meaning because, which introduces a reason and pairs with the Present Simple. Always check which meaning of since is intended before choosing your tense.
✗ Call me when she arrive. · Since he work from home, he saves time. · Wait until the meeting finish.
✓ Call me when she arrives. · Since he works from home, he saves time. · Wait until the meeting finishes.
The Present Simple rules do not change just because the verb is inside a when or since clause. Third-person singular subjects (he, she, it, and all singular nouns) still require the -s/-es ending. Always apply the standard Present Simple agreement rules inside subordinate clauses.