Can I use these different tenses in the same sentence?
Hay muchos personas andando. EDIT: suppose to be muchas
I believe hay is some sort of past tense for hacer and andando is the present progressive for andar.
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Yes, you may have different tenses in one sentence. "Hay muchas personas andando" What you have there is a present tense ("Hay" - "there are) and a present participle ("andando" - going around/walking).
I hope this helps, and ... Come back to the forum soon!