1. In learning a foreign language, it is necessary to advance step by step.
2. When we learn a foreign language, it is necessary to advance step by step.
3. When you learn a foreign language, it is necessary to advance step by step.
4. When they learn a foreign language, it is necessary to advance step by step.
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#1 can be changed into #2. Is that right? What about #3 and #4? Can #1 be changed into #3 or #4?