have been noticed before, a preposition governing a word cannot
be used _after_ the word it governs, as--

The work[204] which I referred to: La obra a la cual referi.

[Footnote 204: Work, artistic, literary, scientific--"obra." Work,
manual, or mental--"trabajo."]

The preposition _con_ followed by an infinitive translates the English
"by," followed by the present participle--

Con ensenar se aprende: By teaching one learns.

The preposition _desde_ refers to "distance of time or space," as--

He marchado desde mi casa: I walked from my house.

However, _desde_ may be used instead of _de_ before names of countries
or cities--

Me escribieron desde Barcelona: They wrote me from Barcelona.

_Para_ may be used together with _con_ = "towards"--

Fue muy bueno conmigo, para mi, _or_ para conmigo.

"_En_ acabando[205] lo hare" has the meaning of "I shall do it as soon
as I have finished."

[Footnote 205: _En_ is the only preposition which may govern a pres.
participle, generally with the meaning given above.]

We add the following idiomatic uses of _Por_ and _Para_ to what we said
about these two prepositions in Lesson XXIX:

_Por_ may translate "on behalf of," "for the sake of," "in favour of,"
"during," "through"--

Hablo por el proyecto de ley: He spoke in favour of the bill.
Se presento por la casa: He appeared on behalf of the firm.
Por la paz y buena armonia concedemos lo que V. pide: For the sake of
peace we allow what you ask.
Les sirvio por cinco anos: He served them during five years.
Trabaja por la manana: He works during (in) the morning.
Vino por Paris: He came through Paris.
Cotizar por un articulo: To quote for an article.
Yo por mi (_or_ por mi parte) prefiero comprar al contado: I, for one,
prefer to buy for cash.
Por[206] rico que sea, no tendra suficientes recursos: No matter how
rich he is, he will not have sufficient means.
Es demasiado avaro por ser tan rico: for such a rich man, he is too
miserly.
Caro por caro prefiero generos ingleses: If I have to pay a dear price,
I prefer English goods.
Por si o por no: In any case, should it be so or not.
Ir (venir) por: To go (come) for.
Enviar por el medico: To send for the doctor.
Por holgazan perdio el empleo: He lost his employment through laziness.
Por bien o por mal: Willy-nilly.
Vendre por la Navidad: I shall come by Christmas.
Por si acaso: In case that.

[Footnote 206: _Por_ has always this mean