I totally agree with

我估計係受你"中文"思維(用詞和表達)的影響

先講英文句子結構
I am的am是verb(或叫原形動詞be) = 我 "是"
I agree的agree是verb = 我 "同意"

中文你或者會在想表達:
對這情況「我是同意你xxx的」
但這想法是把主動想法以第三者的角度想而用客觀的詞組來表達「同意」這結果。英文都有這類客觀講法，但通常當表達主觀感覺或意見時都只會用主觀詞組說的，即是
「我同意你xxx」
因此當講agree一般只會講
I agree with you.
或者
I am in agreement with you. [把agree這個意思變為名詞-再以我的立場am作為動作"同意"(名詞)你] - 是很造作的講法(只會在政治表述或者所謂官腔)才會用。

文法上：
I am verb+ing 是現在進行式的結構，但注意agree這動作是不能continuous(進行中)的，所以世上沒有I am agreeing這說法/寫法的。[同學們很多中了教學例子的毒，以為所有的時態句式都可以像做功課那樣全面複製，現實是任何語文都是按實際"語境"產生語文的，然後分析出語文文法，不是語文的文法像機械複製出句子來。

totally只是adverb形容那動詞(這裏是agree)。
但假如寫I am totally意義是指"我"全個人怎樣了。
例如I am totally confused. I am totally lost.都是被動式詞來表達我的狀況全面地"如何"。

am 之後的動詞會變成 participles (no matter present or past)

I am playing.
I am flattered.

不可以接原形, I am eat apples. 是不合文法的。

因為agree係verb.

你可以當"totally"冇到.
咁你句句子就係:
"I am agree with you."
^ 呢句入面你用左 "am"(auxiliary verb) 同 "agree"(base form) 兩種verb (Godfrey講得好清楚).
簡單d講, 唔可以將兩個verb(或以上)連埋一齊用.

正如我地唔會寫:
"I am go to school."
"I am totally miss you."
"You are totally look like my cousin."
"I am totally understand."
"She is totally ignore me."

我地寫:
"I go to school."
"I totally miss you."
"You totally look like my cousin."
"I totally understand."
"She totally ignores me."

I agree with you (Present simple tense)
I am agreeing with you. (Present continuous tense)
I am agreeable to your suggestion. (agreeable 是 adjective, am 是 linking verb)

文法上沒有 (auxiliary verb) + (base form of verb), 例如 am + agree

2014-03-06 12:46:53 補充：
Cheesy 給了很好例子
如果第三者單數 (He, She, It, teacher) 用 present simple tense, verb 要加 -s
My teacher ignores me.
She does not ignore me.
Does she ignore me?
She may ignore me. (modal verb 後面的 verb 不用加 -s)
(modal verbs 有 can, could, may, might, shall, should, will, would, must)
She does ignore me. (does 加強語氣)