Hell Let Loose: 4 Infantry Tips for Beginners

Leaning
Leaning is a new mechanic released in Patch 2. Leaning can be done by pressing Q and E.
You can use leaning behind trees, and rocks. Leaning will not work if you are prone. So when you are crouching or standing, you can use lean. Also a good tip is to lean against walls, and lean against houses.
Locations to use leaning. Any house on an objective: Like Houses in St. Marie Du Mont. Sometimes you can use them in trenches for close quarters trench combat. Behind trees, fences, hedgerows, and rocks.
Leaning can help you get good spots off at enemies around a corner, it also helps you get a good idea without having to expose your whole body to the enemy or the open. Overall, you can use leaning to get at the enemy before they get at you.

A self explanatory tip is of course cover. Crouching behind rocks, and other objects like trees, houses, tanks, hedgerows, fences, trenches, logs, etc. Hiding behind them makes you safe from direct shots at your location, unless they flank you. Along with using leaning behind cover, you can get an edge against your enemy. Cover already helps, but also using the new lean feature can really help for your own safety.
Cover can be taken advantage by the enemy too, so to counter cover is to use grenades at enemy cover like objects enemies could be hiding behind. Throwing grenades or shooting at logs, trees, houses etc can scare the enemy or kill them if they are behind those cover objects. Or you could just flank them without the enemies noticing their location has been compromised.

Smoke is used to advance forward and also to protect your own self from enemies. But beware, enemies and friendlies will shoot at your smoke. Friendlies will especially if they do not have their markers set up to 500 meters. The smoke is very white and bright. It is blinding for anyone so it is perfect for you. Just be careful because you will not be able to see your enemies either. It is best used to advance from one place to another. As long as they don’t get a lucky shot and kill you through the smoke.

Smoke can be used in open fields, and to move forward in open locations. Artillery can also fire smoke, as well as most roles should have smoke too.

Terrain like hills can be very useful. Rivers, not so much compared to hills anyways. Hide behind hills to protect yourself from the enemy if the enemy is on the other side of the hill. Also if you are on top of the hill, look down at any charging enemies that will be free for you to kill. Snipers can really use this location. Just make sure you can breathe if you get too far up on the high ground. Hills can be used to charge and flank objectives. Hurtgen Forest is a great example, the hill to the east of North Pass is a great location to flank and kill the enemy if you are the Axis.