So I'm currently looking for a new bike and would like to get another mountain bike as it's most important to me that it's resilient and that I can ride all terrain.
However most of the time I'd use it for the city.
Are there mountain bikes (or are there ways for adjusting them) that are more fit for cities? In particular I'd like to sit comfortably on it.
I don't have much expertise with bikes which is why I'm asking here.
The standard advice if you want to ride a mountain bike on the road is to fit slick tires rather than knobby tires, and lock out any suspension if you have it (since you don't need it on the road). That being said, you're going to typically have lower gearing than a road or hybrid bike (due to the nature of mountain biking), though you may have some room to adjust this with cassette and chainring sizes.
At the end of the day, if you're going to be using this bike only on the road, it isn't going to be as nice as an appropriately hybrid or cyclocross or road bike, and it would be preferable not to get a mountain bike. Note that hybrids/cross bikes are often suitable for things like fire roads, gravel and what not (light off road situations); they won't take well to dropping as much as a mountain bike, but you have to ask yourself if this bike is ever really going to do this (and whether you'd be better off just renting a mountain bike when you want to do this, or buying a mountain bike specifically for this purpose).
Hardtail with front suspension lockout. Even if a full-sus has lockout/stiffening on the rear suspension it will add weight at any price point. Slick(er) tyres than normal on a mountain bike will help. The minimum width will be limited by your rims but 35 or 38mm could be an option.
The reason you don't want suspension active while pedalling smooth roads (or even long climbs on dirt roads while mountain biking) is that it is designed to absorb energy from up-down movement. When the main source of up-down movement is your pedalling effort (rather than the trail surface), that's where the energy comes from, making pedalling harder. You'll really see and feel this if you tackle steep climbs on smooth roads.
Mountain bikes can be as comfortable as any other. With slicks they actually ride more comfortably than most because they generally have more cushioning from the tyres.
I have both mountain bike and cruiser. The cruiser is a bit faster and lighter on the road (I have a BSO mountain bike), but in terms of comfort I don't notice a difference. The mountain bike takes bumps/potholes better, skids better and is basically more forgiving for a rough rider on the road and up and down curbs. However with ANY sort of offroad I notice a big difference in favour of the mountain bike.
In saying that, my mountain bike is a 29'er hardtail, I don't feel comfortable on the smaller wheeled mountain bikes, but that might be just a matter of practice. The bigger wheels do make for a smoother more comfortable ride.
I'm not convinced by those that recommend locking out the front suspension. One factor is when commuting you quite often carry loads, I have a toolbelt full of expensive engineering tools on my handlebars and usually a bag and laptop on the back, the front suspension is useful with these things. Commuting is a lot of different needs, speed isn't that big a factor for many people.
Worrying about pedaling effort is good if you're shaving seconds off a race I imagine, but in practice you don't notice it, it's like half filling your water bottle to reduce weight a fraction.
