I read the following sentence today on an easy news web site:
これからはいつも１０秒より速く走ることができるようにしたいです
I've had a look online for an explanation of the end (in bold). I understand that overall it means want to, but I can't get my head around what function です serves here. To me it reads:
"want in order to be able to (run faster than 10 seconds) is".
Is this purely to mark the sentence as polite?
Yes, it's purely a grammatical mark of politeness.
Grammatically speaking, したい (and all other ～たい forms) is an い-adjective, so it takes the same forms as any other い-adjective.
So in plain form:
それは赤い。 "It is red."
それはしたい。 "I want to do it."
And in polite form:
それは赤いです。 "It is red."
それはしたいです。 "I want to do it."
In both cases, the です doesn't add anything to the meaning (the verbal meaning is already part of the adjective form), it just makes the sentence polite. This is perhaps a little clearer in the past tense:
それは赤かった(です)。 "It was red."
それはしたかった(です)。 "I wanted to do it."
これからはいつも１０秒より速く走ることができるようにしたいです - From now on, I always wanna be able to to run faster than 10 seconds(to make a better time at the circuit)
The common patterns for ending sentences in Japanese are:
Verb:
昼ごはんを食べる/昼ごはんを食べます （hirugohan wo taberu/tabemasu) I will eat lunch.
Na-adjective/Noun + だ/です:
彼は静かだ/彼は静かです (kare wa shizuka da/desu) - He is quiet.
just i-adjective or i-adjective + です:
昼ごはんは美味しい/昼ごはんは美味しいです (hirugohan wa oishii/oishii desu) - The lunch is delicious.
The tai form of verbs is treated as an i-adjective, therefore it is an i-adjective ending sentence, so it could be without です, but adding it makes the sentence more polite. Therefore the usage in the sentence you presented.
