Learn Polish: Myślę o nowym smartfonie, ten model jest jednak bardzo drogi. - I am thinking about a new smartphone; however, this model is very expensive.

While you can put jednak at the beginning of a clause, Polish speakers often place it after the first word or phrase for better sentence flow. It works exactly like English when we say 'This model, however, is very expensive.' Placing it after the verb jest makes the contrast sound very natural.

Why does the sentence use nowym smartfonie instead of the dictionary form nowy smartfon?

The preposition o (about) always requires the Locative case. Because smartfon is a masculine noun ending in a consonant, it takes the -ie ending to become smartfonie. The adjective nowy changes to match the noun, taking the masculine Locative ending -ym to become nowym.

Is smartfonie just an English word with a Polish ending?

Yes, smartfon is a direct loanword from English. Polish borrows many modern technology terms but still forces them to follow Polish grammar rules. That is why you must treat it like a regular Polish masculine noun and apply case endings to it, like the Locative -ie seen here.

Could I just say on jest jednak bardzo drogi instead of saying ten model?