making triangles out of the t1 trenches is pretty effective. covers wide area, can have 3 companies shooting in same general area at once, and is godly for cycling depleted and fresh units.
Here's one way you can set em up:



main pros: can cycle, have lots of boolet being fired, good for keeping units in combat while not being injured, if enemy breaks through and begins melee, can move the back 2 units up, if trench network is lost, can easily retake and have good firing arcs.
Firing arc examples:





main downside: arty can suppress all 3 trench pieces at once, and prevents MG spam.

trench connecters are ok at forcing certain networks to have different avenues of escape/counterattack and are good at allowing backline trenches to hold people that can move quickly to frontline trenches

on offence, its literal godsend and will allow you to push.
on defence, its meh. the points likely could have been used better for more MGs, trenches, or maybe a balloon.

if you have an inhumane amount of supply but not much infantry, go ahead and spam i guess. otherwise, use either to suppress or siege. siege batteries are good for attacking big armies (aka whatever tf is in Ypres/Ghent), but otherwise are super expensive and heavy on supply usage. don't use em unless you REALLY wanna force a major battle.
if you got gas, use it. gas damages infantry in trenches a lot in the early to mid war, and is better to use than the precision barrage in most cases. sadly gas no make suppressed, but who needs suppression if all your enemies are ded. note that gas is only really useful once you tech chlorine.

storage depots can make or break a battle, make sure you have em anywhere you think a bigass battle will happen, or on tiles you plan on attacking from. more supply = more dakka = more dead enemy.

other buildings like hospitals and the canteens are also good, but imo aren't as good as storage depots. just about every frontline region should have at least lvl 1 supply depot by 1916

in battles where u outnumbered, its best to try and hold onto a point and your HQ. if they're close by, i'd recommend having a few connected networks to defend said point and HQ.

if everything is spaced out, make a U network facing da enemi at each point (HQ as well), and spam as many triangles as you can afford onto the points. most of the time your HQ should be pretty safe even with lackluster defenses, especially on defense while holding 2 points. the AI loves swarming the points, so forcing them to lose 10s of thousands of bois tryna take 1 trench is the best way to grind them to a halt and cease fire.

for attack, just dont unless you have plenty of supply and arty and men to lose. spam arty to suppress, have bois pushing in so that they get in the trench right as the arty stops. personally i'd recommend keeping goals small, like taking a trench network or if you're lucky, a point. once you get your goals achieved try to ask for cease fire unless you'll have a loss. the citizens do not accept Ls, so unless its a stalemate or W, keep going if possible.

da more men on your side that die, the lower national will gets, aka MOAR MISERY and less nice things happening. if you can, try to keep national will above 600. below 600 is where the chaos begins, so try and get the cringelords you're fighting to have low national will. aka slaughter the conscripts/riflemen until the enemy's people are too depressed to want to fight. oh yeah, keep your people alive in battles (duh). if you got units that are super weak and are near a reinforce point, have them withdraw to not get deaded, then replace em with fresh lads. more units stay alive and happy = more medals when battle over = LESS MISERY = MORE MONEY/SUPPLIES/NATIONAL WILL!

planes are cool. big boom is also cool. combining them gives you big flying dakka that insta snipes MGs and arty. 10/10 would boom again. fighters are also super good once you get the tech for hunting balloons, it makes you become John Cena with how little the enemy can see.

more plane = more Ws = happy civilians = less misery