if i begin a trade on my laptop can i complete it on my mobile phone?

Yes, you can. I trade with Trade Nation & FxPro and I am able to use their mobile app to monitor trades opened on MT4/5 desktop.

One day i was using my laptop to trade with the MT5 platform when my laptop suddenly crashed. I quickly logged in to the brokers mobile app on my phone and kept monitoring the already open trade till it was profitable enough then i closed it.

I can relate, the mobile app is a life saver! so many times when i couldnt access my laptop due to network outage, i used to mobile app to manage the trades- it happened even today.

Normally the network provider for my laptop data is different from the network provider for my mobile phone so even if one is down, chances are the other one will be up.

any trade you start on your computer will also reflect on your mobile trading app as far as you logon to the same trading account on both laptop and mobile app

Yes, you can! and this is why it is important to be with a broker that offers a solid mobile app.

Yes desktop trading platforms like MetaTrader also have mobile apps, so you can manage a trade you initiated on the desktop from your mobile app.

Yes, you can but there are a few things to note: if you opened a cTrader account you may need to download your brokers version of the cTrader mobile to be able to manage your trade from your phone. In fact, some brokers dont allow you manage trades for MT4 and MT5 accounts on their mobile app (one of such brokers is Trade Nation) so in that case, you must download the MT4/5 mobile app to be able to manage your trades.

I concur to this, I once opened an MT5 account with a broker and was surprised when I was told I couldnt use the brokers mobile app to place trades and I had to download the MT5 app which was boring to use.