Can someone explain to me the reinforcement system?

I was just wondering, why is it when sometimes ill capture a neutral/player city and i get no reinforcement. (not adding reinforcement "week") other times ill get reinforcements and sometimes i wont. why is that? i heard it was when u attack with tanks. is that true?

You will be able to build reinforcements in a captured city (that you hold the capital, of course) in any week if your enemy hasn't done it yet. Also, if he bought 4 units in a 8 reinforcement city and you capture it you will still be able to buy 4 more additional troops. The only exception is when you capture a city of a late joiner, even if he didn't bought all his possible units you will only be able to hire the default number of reinforcements.

Thank you. so its the total reinforcements in the city minus the amount my enemy bought got it. what about neutral cities? because sometimes ill capture it and there's no reinforcements.

The only way I can see a neutral city not having reinforcements is if it was controlled by a player who left and used all the production from that city before they left. Or a its glitch.