Can you trademark an ordinary dictionary word?

Yes, when the word has nothing to do with the goods. APPLE is an ordinary word and a very strong trademark for computers, because it describes nothing about a computer. The same word would be unregistrable for fruit.

What is a crowded field?

A field in which many different owners already use the same term in their marks. It does not stop you registering, but it shrinks how much protection any one owner gets, because buyers have learned to distinguish those marks by their small differences.

What if my word is descriptive of what I sell?

Then it is refused under 2(e)(1) unless you can show the public has come to treat it as a brand rather than a description, which normally takes years of use and real evidence. The alternative is the Supplemental Register, which offers far less.

Can I trademark a common word if I add my own twist?

Deliberate misspellings rarely help, because marks are compared by sound and a misspelt descriptive word is still descriptive. Adding a genuinely distinctive second word usually does more, though what you then own is mostly the combination.

Can you trademark a common word?

The question is asked the wrong way round. Whether a word is common has almost nothing to do with whether you can own it. What matters is the relationship between that word and what you sell.

APPLE is about as common as English gets. It is also one of the strongest trademarks in the world, because an apple has nothing to do with a computer. The same word, applied to fruit, is unregistrable by anybody, forever.

The spectrum, applied to your goods

Every mark sits somewhere on a five-rung ladder, and the rung is set by the word plus the goods together, never by the word alone:

Generic for your goods. Never registrable. You cannot own the word people need in order to ask for the thing.
Descriptive. Refused under 2(e)(1) unless you prove acquired distinctiveness, which normally means years of use and evidence that buyers treat it as a brand.
Suggestive. Requires a mental step to get from the word to the product. Registrable straight away, and the sweet spot for most new businesses.
Arbitrary. A real word with no connection to the goods. Strong.
Fanciful. Invented. Strongest, and the easiest to clear.

So the question to ask about your common word is not how common it is. It is: does this word describe, or even hint at, what I am selling? The further from the product, the easier the path. The full version of this argument is in the distinctiveness spectrum.

The crowded field problem

Common words attract a second issue that has nothing to do with descriptiveness. If many different owners already use the same term in that field, the term sits in a crowded field, and every mark containing it gets narrow protection.

This cuts both ways, which is why it is worth understanding rather than fearing. The earlier owner cannot easily stop you, because their own scope is narrow and buyers already distinguish a dozen similar marks by small differences. But you will not be able to stop the next entrant either. You share the word with everyone already using it in that field.

Whether that is acceptable is a business decision. If the name only needs to identify you, it may be fine. If you expect to police it, it is a poor foundation.

What actually helps

Pair it with something distinctive. A strong second word does real work. What you then own is largely the combination, so keep using them together.
Pick a word far from your goods. The cheapest strength available, and it costs nothing at the naming stage and a great deal later.
Narrow your identification of goods. A precise description reduces the overlap an examiner can find against an existing mark.
Do not rely on a misspelling. KWIK is QUICK for these purposes. A misspelt descriptive word is still descriptive, and you have added an ongoing cost in explaining the spelling forever.

The short version

Common words make excellent trademarks, on one condition: they must be common somewhere other than your own aisle. Check the word against your goods rather than against the dictionary, and check whether the field is already crowded, and you will know most of what you need before filing anything.