to be

你所問的句子應該寫成：
She describes herself a singer. (她形容自己是一名歌手。) or
She wants to be a singer. (她希望成一位歌手。)

Describe 本身就可以take 兩個objects (herself, a singer), 無須要用 to be 或其他 preposition 連接。

此外，請你搞清楚，"to"和"be"是兩個不同的部件，屬於不同的Parts of Speech, 在一個句子裡亦各有不同的功能。它們走在一起只是功能上的需要，並不是"to be" 加起來有一個甚麼特別的意思/用法。

先講 "be"，它是一個Auxiliary Verb，是 is/am/are 的原形 (basic form)。一句英文句子裡面一定要有一個 verb，就是當你想不到任何一個動詞時，都要放個 "be"落去。例如：I am.

至於 "to"，它可以是preposition(前置詞)，adverb(副詞)或 infinitive (不定詞)。

Preposition 的作用是顯示一個句子裡兩個或以上的不同部件的關係。例如：
I go to school in the morning.
He sits on the chair.
The height of the building is 382m.

Infinitive 則比較複雜。在英文文法裡，當一句句子須要出現第二個動詞時，它不能就咁跟著另一個動詞，例如我去游泳不能寫成 I go swim. 這時候第二個動詞就要 +ing 變成 gerund (即 I go swimming.)；或者以 to 來帶出第二個動詞(即: I go to swim.) 這就是 Infinitive. to be 正是一個infinitive.　前面的 She wants to be a singer. 正是一例。

至於He seems to be fine. 裡面的 "to be" 其實可省略不要的(即是沒有作用)，即 He seems fine 己經符合文法的了。這可能是口語習慣導致有這情況。但 seems 亦可以用 infinitive 的，例如：He seems to be doing fine today.

您好，我推薦您一個非常不錯的英語學習網站，里面有很多非常不錯的學習資源，您能從中得到不少幫助：

http://www.hkenglishstudy.info

祝您好運!

she describe herself to be a singer SHE DESCRIBED HERSLEF AS A SINGER = SHE MAY NOT BE A SINGER BUT SHE TELLS OTHERS THAT SHE IS A SINGER.
OR
SHE WANTS TO BE A SINGER or SHE IS GOING TO BE A SINGER

she describe herself that she is a singer

Both sentences do not sound right so see my example above please.

如果已經係singer, = she is a singer, she is already a singer

he describe herself as a singer

但用to be 果句 得吾得?

he seems fine

he seems to be fine

These two sentences are actually the same but expressed differently.

"she describe herself to be a singer"

呢句好似唔係幾make sense喎!!
how about:
she wants to be a singer or she going to be a singer

如果已經係singer., 點解唔直接講:
she is a singer or she says she is a singer.

he seems fine 佢幾好吖

he seems to be fine 睇嚟佢應該冇咩事

「攪錯唔好鬧我呀」

to be基本上表達「是」，沒有「未來」或「將會」意味，但可能常跟going連用，going to be就是「將來是」「將會是」，所以是going有「未來」之意。

She describes herself to be a singer
她表示(形容)自己是一個歌手
She wants/wishes/likes to be a singer
她希望/願望/想望是(成為)一個歌手
[只是我們會意她現在一個歌手，但這句是一句我的志願statement，所以作為一個歌手是沒有時限的]

He seems to be fine.
他看來無羔。[並沒有未來之意]

He seems going to be fine.
他看來將會康復。[有現來未康復，要將來一段時間才能完全復原，但是是積極的。]