Tengo sed hoy.

In Spanish, thirst is usually expressed with tener + a noun:

• tener sed = to be thirsty
• literally, to have thirst

This is one of several very common Spanish expressions that use tener where English uses to be:

• tener hambre = to be hungry
• tener sueño = to be sleepy
• tener frío = to be cold

While sediento/a exists, it is much less common in everyday speech. The normal, natural way to say it is tengo sed.

Sed is a noun meaning thirst.

In this expression, Spanish normally does not use an article:

• Tengo sed
• ¿Tienes sed?
• Tenemos sed

That is just the standard fixed expression. Saying tengo la sed would usually sound unnatural in this basic sense.

Spanish often leaves out subject pronouns because the verb ending already shows who the subject is.

• tengo = I have

So yo is not necessary unless you want emphasis or contrast:

• Tengo sed = I’m thirsty
• Yo tengo sed, pero ella no = I’m thirsty, but she isn’t

In normal conversation, dropping yo is very common.

Yes, hoy can move around.

Common options include:

• Tengo sed hoy.
• Hoy tengo sed.

Both are correct. The difference is mostly about emphasis:

• Tengo sed hoy puts the main statement first and adds today
• Hoy tengo sed highlights today more strongly

In everyday speech, both sound natural.

It is grammatically possible, but it is much less natural in most everyday situations.

Spanish usually prefers:

• Tengo sed hoy
• Hoy tengo sed

Putting hoy between tengo and sed can sound awkward unless there is a special rhythm or emphasis.

You can simply raise your voice in speech, or use question marks in writing:

• ¿Tengo sed hoy? = Am I thirsty today?

But if you are asking someone else, you would change the verb form:

• ¿Tienes sed hoy? = Are you thirsty today?

Spanish does not usually need a helper verb like do.

Put no before the verb:

• No tengo sed hoy. = I’m not thirsty today.

This is the normal Spanish word order for negation:

• no + verb

Examples:

• No tengo hambre.
• No tengo sueño.

You change the form of tener:

• Tengo sed hoy = I am thirsty today
• Tienes sed hoy = You are thirsty today
• Tiene sed hoy = He/She/You (formal) is thirsty today
• Tenemos sed hoy = We are thirsty today
• Tienen sed hoy = They/You all are thirsty today

The noun sed does not change.

Sed is feminine:

• la sed

But in the expression tener sed, you usually do not use the article, so learners often do not notice its gender at first.

If an adjective is used with it, the agreement is feminine:

• mucha sed = a lot of thirst
• sed intensa = intense thirst

In most Latin American Spanish, sed is pronounced roughly like sehd, with:

• se- like seh
• final d softer than in English, especially in casual speech

And the whole sentence:

• Tengo sed hoy

is approximately:

• TEN-go seh(d) oy

A few notes:

• tengo has the stress on the first syllable: TEN-go
• hoy sounds like oy in English boy

Yes. A very common expansion is:

• Tengo mucha sed hoy. = I’m very thirsty today. / I have a lot of thirst today.

Other natural examples:

• Tengo bastante sed hoy. = I’m pretty thirsty today.
• Tengo un poco de sed hoy. = I’m a little thirsty today.

So sed works well with quantity words.

Yes, native speakers would say it, and it is completely natural.

That said, if today is not especially important, they might simply say:

• Tengo sed.

If they want to emphasize that it is unusual or specific to today, then:

• Hoy tengo sed
• Tengo sed hoy

both work well.