How do brokers match buyers and sellers using the bid/ask prices? Why are orders sometimes partially filled? How does that happen? Is there a way to avoid the double fees?

I have a sub question. If the orders are split across various timings of the day, eg out of 1000 shares, 500 is bought in the morning but rest 500 is bought in the afternoon, is it just one commission?

Does it make a difference if i put in a buy at 500 shares in the morning and it got throught, then i separately put in another 500 shares in the afternoon and it got through in that same day?

ps im using poems

Yes. If partial order is filled, it is considered 1 trade, and you will be charged full commission.

If you want to buy/sell the 1000 shares without partial order then you have to buy up or sell down. If you queue then you risk the partial order.

This usually happen when people want to try and save on that 1 bid and queue at a better pricing to save on that little bit but end up partial order.

Another way to avoid partial order is DON'T trade low liquidity stocks.