Tämä johtolanka oli hyödyllinen.

It follows a very common Finnish pattern:

• Tämä johtolanka = the subject
• oli = the verb
• hyödyllinen = a predicate adjective

So the sentence is:

• This clue
  • was
    • useful

In grammar terms, this is a copula sentence: the verb olla (to be) links the subject to a description.

Tämä means this. It is a demonstrative word used to point something out.

Here it modifies johtolanka, so:

• tämä johtolanka = this clue

Finnish does not use articles like a or the, so tämä is doing the job of specifically pointing to one clue.

A few related words are:

• tämä = this
• tuo = that
• se = it / that / sometimes the in everyday speech

Because johtolanka is the subject of the sentence, and the subject of a basic Finnish sentence is usually in the nominative.

So:

• tämä = nominative singular
• johtolanka = nominative singular

Together they form the subject:

• Tämä johtolanka = This clue

If the word had another role in the sentence, it could take another case, but here it is simply the thing being talked about.

Johtolanka means clue, especially in the sense of something that helps you solve a mystery, understand a situation, or find the answer.

It can often feel like lead, clue, or hint, depending on context.

A related word is:

• vihje = hint / clue

Very roughly:

• johtolanka often feels like a clue or lead that helps you proceed
• vihje can be a hint more generally

In many situations, both can be translated as clue.

Oli is the past tense of on, which comes from the verb olla (to be).

So:

• on = is
• oli = was

That is why:

• Tämä johtolanka on hyödyllinen = This clue is useful
• Tämä johtolanka oli hyödyllinen = This clue was useful

Finnish does not need a separate word for did or anything like that here; the past meaning is built into oli.

Hyödyllinen is an adjective meaning useful. In this sentence it is in the nominative singular, because it is a predicate adjective describing a singular nominative subject.

So:

• johtolanka = singular subject
• hyödyllinen = singular predicate adjective

This is the normal form in a sentence like this.

A useful comparison:

• Tämä johtolanka oli hyödyllinen. = This clue was useful.
• Nämä johtolangat olivat hyödyllisiä. = These clues were useful.

Notice that in the plural, the adjective changes too.

Because with a normal singular subject like tämä johtolanka, the predicate adjective is usually in the nominative, so you get:

• hyödyllinen

The form hyödyllistä is the partitive form, and it is used in different situations.

For example:

• Se oli hyödyllistä. = That was useful.

Here se refers more generally to a situation or action, so Finnish often uses the partitive adjective.

But with a concrete singular noun as subject:

• Tämä johtolanka oli hyödyllinen.

So this sentence uses the normal nominative predicate adjective.

It is a -nen adjective, which is a very common Finnish word type.

The dictionary form ends in -nen:

• hyödyllinen = useful

These words change in predictable ways when declined. For example:

• nominative: hyödyllinen
• genitive: hyödyllisen
• partitive: hyödyllistä
• plural partitive: hyödyllisiä

So if you learn that hyödyllinen belongs to the -nen type, it becomes easier to recognize and inflect it.

Often, yes. Finnish adjectives usually agree with the noun in number and case.

In this sentence, the adjective is not directly attached before the noun; it is a predicate adjective after the verb. Even so, it still matches the subject in the normal way here:

• singular subject → hyödyllinen

Compare:

• hyödyllinen johtolanka = a useful clue
• hyödylliset johtolangat = useful clues
• Tämä johtolanka oli hyödyllinen.
• Nämä johtolangat olivat hyödyllisiä.

So agreement is an important pattern to notice.

Yes. Finnish word order is fairly flexible, although the most neutral order here is:

• Tämä johtolanka oli hyödyllinen.

Other orders are possible for emphasis or style, for example:

• Johtolanka oli hyödyllinen. = The clue was useful.
• Hyödyllinen tämä johtolanka oli. = This clue was useful.
  This sounds more marked or emphatic.

A learner should usually start with the neutral order:

• subject + verb + description

That is the safest and most natural basic pattern.

Just change oli to on:

• Tämä johtolanka on hyödyllinen. = This clue is useful.

Everything else stays the same.

So:

• on = is
• oli = was

This is one of the most useful verb pairs to learn early.

Use the Finnish negative verb ei and the special form ollut:

• Tämä johtolanka ei ollut hyödyllinen. = This clue was not useful.

Compare:

• on → ei ole
• oli → ei ollut

So the past negative is not formed with a word meaning not was; Finnish uses its own negative structure.

The plural would be:

• Nämä johtolangat olivat hyödyllisiä. = These clues were useful.

Changes:

• tämä → nämä = this → these
• johtolanka → johtolangat = clue → clues
• oli → olivat = was → were
• hyödyllinen → hyödyllisiä = useful → useful (plural form)

This is a good example of how several parts of the sentence change together in Finnish.

A rough pronunciation guide is:

• Tämä ≈ TA-ma
• johtolanka ≈ YOH-toh-lahn-ka
• oli ≈ OH-li
• hyödyllinen ≈ HUU-y-duel-li-nen

A few important points:

• j in Finnish is like English y in yes
• ä is not the same as English a; it is more fronted
• double letters are pronounced longer, so ll in hyödyllinen is held a bit longer
• Finnish stress usually falls on the first syllable

So the rhythm is:

• TÄ-mä JOH-to-lan-ka O-li HYÖ-dyl-li-nen

Yes — several very common building blocks:

• tämä = this
• oli = was
• hyödyllinen = useful
• the pattern X oli Y = X was Y

This pattern works with many other words:

• Tämä kirja oli kiinnostava. = This book was interesting.
• Tämä idea oli hyvä. = This idea was good.
• Tämä ratkaisu oli hyödyllinen. = This solution was useful.

So this sentence is a very good model for making many similar Finnish sentences.