en_tn_condensed/act/20/31.md

be on guard

"be awake and alert" or "watch out." Christian leaders being alert about anyone that may harm the community of believers is spoken of as if they were guards in an army watching out for the enemy army.

Remember that

"Continue to remember that" or "Do not forget that"

for three years I never stopped warning ... night and day

Paul taught them over the space of three years, but not necessarily every day for three years.

I never stopped warning ... you

"I warned ... you as often as I could"

with tears

Here "tears" refers to Paul's crying because of the strong emotion of concern he felt while he was warning the people.