i can swim.

首先，甚麼是 FINITE 和 INFINITE VERB？
FINITE VERB 是跟 TENSE 轉，如 CAN， SWIM，大部分動詞都是。（SWIM 在這裏是 BARE INFINITIVE，但在其他地方可跟 TENSE 轉）
A verb form the use of which which is limited by subject and tense, e.g. I go, he goes, she went, they have gone.INFINITE VERB 是不跟 TENSE 轉，如 OUGHT, MUST

A verb form the use of which is unrestricted by subject or tense. In English this means the infinitive, the gerund and the participle.

http://www.davidappleyard.com/english/grammar.htm

這句好簡單：

i can swim.

SUBJECT ＋ FINITE VERB ＋ BARE INFINITIVE。

INFINITIVE 有兩種，加 TO 和不加 TO。 加 TO 的叫 TO INFINITIVE， 不加的叫 BARE INFINITIVE，都是用 VERB 的 BASIC FORM，不轉 TENSE，因之前的 AUX VERB CAN 已經看到個 TENSE，是 PRESENT TENSR。要轉都是 CAN 轉，後面的不轉。

上面位說 SWIM是 Base - Imperative。錯 Here, "swim" is not an imperative. Imperative 是命令式第二身，這裏是第三身，怎會是 IMPERATIVE？！
圖片參考：http://hk.yimg.com/i/icon/16/6.gif

Base - Imperative Work well!Make this.Have a nice day.Be quiet!
http://www.englishclub.com/grammar/verbs-verb-forms_main.htm

2006-12-12 23:03:48 補充：
INFINITIVE 是 NON FINITE／INFINITE VERB， 所以 SWIM 在這裏作為 BARE INFINITIVE 是 NON FINITE／INFINITE VERB。