I'm having difficulty controlling the Hastati light infantry in Rome TW. Are they more suited for standing put or attacking? When should I use guard mode? Why do they always throw their spears first when attacking (slowing them down)?
You can read through this thread to get some other people opinions, but here is how I usually use them:
The Hastati are very diverse and can be used offensivley or defensivley. Your best bet is to use them defensivley as a front line. The Hastati are very effective as a defensive line. They will comprise your front line for infantry on infantry combat, (of course phalanxes comprising your front line for calvary combat). If you set them to defensive then they will volley when the enemy gets in range. You can sometimes get off a few good volleys before the two forces collide. Once your Hastati have tired then start moving in your higher class infantry so they will be fresh for combat. If you are using them to flank/charge then hold down ALT and click attack. They will not throw their spears and just charge in. This saves some time.
Also, to answer your second question as to why do they throw their spears. Thats just how it was done back then. You can read the TW Wiki for some more information.
Hastati are competent early-game infantry and will probably make up the bulk of your front-line troops until you are able to build the slightly more expensive and better armoured Principes. They are armed with two weapons - short-range javelins that they will throw at units just before charging, and swords for general hand-to-hand combat.
The javelins are quite powerful, but sometimes slow the unit down at inconvenient moments (e.g. if the enemy is charging you). You should be able to hold down the Alt key to force a charge without the ranged attack, if one is needed.
Overall the Hastati are quite flexible and reasonably well suited to both charging enemy infantry and standing firm, but they're vulnerable to cavalry charges and to the heavier infantry - especially that of the Carthaginian and the Greek factions. Once you have the more advanced Principes and Triarii, Hastati may be better used for flanking the enemy or pinning them down while you line up attacks from the side or rear.
Early game Hastati are the core regular line infantry in game until you secure heavier Principe and Legion cohorts. Their role historically in early armies was to form the main battle line between the Velites ((skirmishes) and Triarii (veteran spear armed third line), armed with pilum (a specialised throwing spear with a head designed to drag down shields and injure incoming attackers) and the short sword, or gladius, for thrusting. While lightly armoured these men fight in close formation with locked shields. They are most effective when formed up defensively or charging after missile discharge at the enemy.
In game I find they work best if formed in a slightly deeper formation, this gives them more staying power and prevents the unit breaking open under pressure. As 'light medium'infantry these troops can manoeuvre quickly and are particularly effective if used to pin and flank an enemy or pin for friendly flanking cavalry. They should only face cavalry at the halt and in formation but won't last long without help. It's sometimes best to turn off their missile fire to avoid friendly fire and uncontrolled volleys. Use them extensively as they are the easiest line infantry to replace and replenish and should be the core of any legion army supported by cavalry, archers and skirmishes.
The best way in which to use hastati and any unit for that matter is spreading them out. Having your main core units attack in one huge group is inneffective and will force them to route since they will lose moral very quickly. If each sing soldier is fighting hand to hand combat in each Hastati unit, you will force the enemy to route especially if it is against italian spearmen. I'd recomend upgrading your fields of mars barraks early in the game so you can access the much stronger principas and triarri.