Learn Hardcore Icelandic: Ég held að einhver sé í garðinum.

The infinitive form is að halda = to think / to believe / to hold.
held is the present tense, 1st person singular:
- ég held – I think
- þú heldur – you think
- hann/hún/það heldur – he/she/it thinks
- við höldum – we think
- þið haldið – you (pl.) think
- þeir/þær/þau halda – they think

So Ég held is the natural way to start a clause meaning I think (that)….

Why do we need að after Ég held? Is it like English that?

Yes. að here is a subordinating conjunction, very close in function to English that.

Ég held að… = I think that…
It introduces a subordinate clause: að einhver sé í garðinum (that someone is in the garden).

Just as English that can often be dropped, spoken Icelandic sometimes drops að, but að is more often kept than English that. So:

Ég held að einhver sé í garðinum – fully normal and clear.
Ég held einhver sé í garðinum – possible, more speech-like.

Why is it sé and not er? What form is sé?

sé is the present subjunctive of vera (to be).

er = present indicative (factual, neutral): einhver er í garðinum – someone is in the garden.
sé = present subjunctive (non‑factual, uncertain, reported, wished, etc.): að einhver sé í garðinum – that someone be / is (maybe) in the garden.

After verbs like:

ég held – I think (not 100% sure)
ég vona – I hope
ég óttast – I fear

Icelandic normally uses the subjunctive, because the statement is not presented as a bare fact, but as a belief, hope, fear, etc. That’s why you see sé here.

Could I say Ég held að einhver er í garðinum instead, using er?

You will sometimes hear er in everyday speech, especially from non‑careful speakers, but:

Standard, correct form: Ég held að einhver sé í garðinum.
Using er here is generally considered non‑standard or less correct.

So if you’re learning the language, treat Ég held að einhver sé í garðinum as the form to imitate.

What does einhver mean exactly? Is it someone or anyone?

einhver is an indefinite pronoun that can translate as:

someone
anyone
sometimes a certain person, depending on context.

In this sentence, einhver most naturally means someone:

Ég held að einhver sé í garðinum.
→ I think (that) someone is in the garden.

About its form:

Nominative singular: einhver (for masculine and feminine), einhvert (for neuter).
It declines for case, number and gender, but in this sentence it’s nominative singular, the subject of sé.

Why is it í garðinum and not just garður or something like that?

í garðinum breaks down like this:

í = in (a preposition)
garðinum = the garden in the dative singular.

Two key points:

í can govern either accusative (motion into) or dative (location in):
- Ég fer í garðinn. – I go into the garden. (accusative: garðinn)
- Ég er í garðinum. – I am in the garden. (dative: garðinum)
In sé í garðinum, the meaning is location, so dative is used.
garður is the base noun (a garden).
garðinum is definite dative singular = in the garden (a specific garden already known from context).

So í garðinum literally means in the garden, not just in a garden.

How is the word order inside the að‑clause decided? Why einhver sé í garðinum?

The að‑clause here is:

að [einhver] [sé] [í garðinum]

Inside this clause, the word order is basically Subject – Verb – Rest:

Subject: einhver (someone)
Finite verb: sé (is, subjunctive)
Prepositional phrase: í garðinum (in the garden)

So the structure is regular:

Ég held (main clause: subject + verb)
að einhver sé í garðinum (subordinate clause: subject + verb + complement)

The main difference from English is not the order, but the verb form (sé vs. English indicative is).

Can I add það and say Ég held að það sé einhver í garðinum? Does it change the meaning?

Yes, this is possible and grammatical:

Ég held að það sé einhver í garðinum.

Here það is an expletive / dummy subject, similar to English there in there is someone in the garden.

Nuances:

Ég held að einhver sé í garðinum.
Focus: on some particular “someone” who is in the garden.
Ég held að það sé einhver í garðinum.
Focus: on the existence of someone in the garden (there is someone or other in the garden).

Both are fine; the original sentence without það is very natural.

If I put the sentence in the past, what happens to held and sé?

For a past‑time meaning, both the main verb and the verb in the að‑clause usually shift:

Present:
Ég held að einhver sé í garðinum.
→ I think (now) that someone is (now) in the garden.
Past:
Ég hélt að einhver væri í garðinum.
→ I thought (then) that someone was (then) in the garden.

Changes:

held → hélt (past of að halda)
sé → væri (past subjunctive of vera)

The use of the subjunctive continues, but in its past form (væri).

How is this sentence pronounced? Any tricky sounds for English speakers?

Approximate pronunciation (IPA and rough English hints):

Ég – /jɛːɣ/
- Starts with a y‑sound /j/.
- Final consonant is a soft, voiced fricative /ɣ/ (like a soft, voiced kh).
held – /hɛlt/
- Clear h, then e like in get.
að – /að/
- Final ð is like the th in this.
einhver – /ˈei̥n̥kvʏr/ (varies by speaker)
- ein‑ roughly like English ane in aneurysm.
- hv can sound like kv or hw depending on dialect; ʏ is a short, rounded vowel between u and i.
sé – /sjɛː/
- sj is like sh but more s‑like; é is long, close to yeh.
í – /iː/
- Long ee.
garðinum – /ˈkarðɪnʏm/
- g here is hard /k/.
- ð again like English th in this.
- Final -um is a short, reduced vowel sound.

Said smoothly:
[jɛːɣ hɛlt að ˈei̥n̥kvʏr sjɛː iː ˈkarðɪnʏm]

Is Ég held always used for I think? What about Mér finnst?

Both exist, but they’re used a bit differently:

Ég held að… = I think / I believe that…
- More about belief, assumption, opinion.
Mér finnst að… (or just Mér finnst…) = literally It seems to me that… / I feel that…
- Often used for personal impressions, subjective opinions (how something seems, feels, looks to you).

In the sentence you gave, Ég held að einhver sé í garðinum is the natural choice: you are stating what you believe about a factual situation.